First of all.. What are browsergames? They are most often some text-based games with some images you can play straight from your browser. There are some games like Travian (I will reference this game later, so check at least the screenshots out), OGame, or to name a completely different one: Runescape. You can play all these games in your browser, which qualify them for being a browsergame. Most of these games don't require any plugins, as for example Runescape does (it's still a Java applet), so I will discuss these, which just require the integrated browser renderer.
As most of the guys of "the scene" know, there are a lot of strategy games. In fact, you really have to search to get to play an RPG or any other type. The main reason might be that the strategy games do not only prove to be very easy to create (think about it: some units, some map system, and voila, it's playable), but they also are somehow the king of addictiveness. My girlfriend for example tried several RPG browsergames until she just stuck to Travian, as it's much more immersive as the former. I'd say she even got lucky to see the RPG games first, as now she will be probably lost in the whole mix of strategy games. To get yourself started: Legend of the Green Dragon, Duels and Gladiatus. This is kinda sad for myself, as I'm a huge fan of RPGs. One thing though is, that most of the strategy games are in outer space for some reason, which is awesome, so I'd say touche. (I won't go into much detail to these, as they are there en masse.)
There are also games like football, basketball managers, or stock investing games. I have not so much experience with those, but maybe some day. My favorite of the nonstandard bunch is Slavehack. Make sure you check it out.
Having played lots of different games and analyzed them, I can provide a small list of what makes games addictive and what makes them just plain good. I made this list with help from some friends, so it's not purely my own opinion. Keep in mind, that these points are meant for game developers, or probably those who want to start developing these kind of games.
What is addictive?
1. The simulated passiveness.
If you are a vivid player of these games, you may see that all these pages are saying that you can play when you want and how often you want, so you can play it just when you are hungry for some quick game. Together with some ridiculous build times for buildings and units if you compare with realtime strategy games (Warcraft 3 ~1min, Travian ~30min, other games go even as far as to caculate build times in days), you have a truly passive game. However, as you may notice, there are only few players who actually play it this passive way. Most of the people I know just sit around all day pressing F5 or start integrating themselves into a community. Your just plain passive game gets absolutely active. Once somebody else is say, attacking your base, from a simple 20min building plan you may start to use these resources to add some more soldiers. After the fight you start to build again. If you haven't got the logic behind it: A player may wait 20 minutes, but he checks every 5 minutes if he should change his plans to something more defensive or not.
Now you have the completely unintentional effect of your browsergame being actually active. And here lies the trick: The user thinks he can play where and when he wants, but in reality he is constantly thinking about how to defend himself on a possible attack and integrate into the guild more to get a good defense shield, whatsoever. This is of course only an example, but it already tells how far a game can go.
2. PvP and Social Networking
Most of the browsergames include some kind of PvP system. This is most of the time even the essential part of getting forward, even playing the game. Strategy games emphasize creating guilds and raid other guilds and their bases, and even RPG games these day must include some form of PvP. The kinda addictive game Duels mentioned earlier is on a complete PvP basis, as there are only a few bots to fight against, and the rest are players to get your experience from. This creates, contradictive to the name "Player versus Player", a stronger bond in the community, and you have a sense of progress and may get a ridiculous amount of self-esteem. However, PvP does not only mean fighting against each other. There are a lot of forms of PvP.
- Trade. A good trade system can make the difference between sucky game and good game. As far as trades go, they have to be easily accessible (maybe only with an upgrade to mark the status of entering the community?), and easily understood. Some games seem to implement a trade fee to stop swapping items back and forth, but this just hinders the gameplay, and you just often see in those games that the trade system does not flourish as good as it could have. A suggestion of mine is to make user created items of high value to keep everyone interested, but more to that later.
- Leaderboards/Highscores. This is probably the oldest form of PvP. Who doesn't remember the unbeatable highscore in the arcades from yesteryear? This concept got in the fast few years more popular, as it was lost for some time, but came back with huge success. People love leaderboards, and the more you have the better. Have a overall leaderboard. Have a leaderboard for all classes/races you have in your game. Have a leaderboard for your guild. People love them. Give them what they want. Just make sure it's really not just one, as a big leaderboard with 100.000+ users can simply scare people away.
- Ego. There is always the factor of "my stuff is better than yours", and the more diversive your items are, the harder the good items can be obtained, the better this effect is. However, they should still be obtainable, we don't want implement a hard scripted AI fight, just to have everyone get their army killed there. There is also something, which can be easily implemented into any game, and can even make lots of fun to do: Ranks. Say if you have enough experience you gain the rank "soldier", or if you complete a special event you gain the rank "walrus". It can be completely nonsense, but if you have a player asking another player how to get that rank and if he actually starts questing for that rank, the game designer wins.
- Fight. This was already discussed, but it's for most people understood as the primary form of PvP, if not the single one, which is kinda sad.
3. Clever backstory, but...
Most of the games have a quite complex and interesting story behind them, but lets be honest. Who reads that stuff? You can do that when you're waiting to get some buildings done, but even then you won't spend more than 5 minutes on a nice, but several tome long story. Luckily, most games seem to understand this and offer a nice alternative: Have a story, but don't force the player to know it, or even understand it. Gameplay first, and the story should just support it. Give the player the choice to read/understand the story, or just play the game. We want something addictive, not something that needs to be read first. RPGs can get a bit deeper than strategy games for example, but that's only because of their storytelling-like nature. In the end of the day the game with the easiest to pick on gameplay wins, so don't pay a writing team if you can get some additional programmers to make your game even better.
4. Customization, or just standing out of the crowd
When you play a game, do you want to be like a sheep, the one which is just like all the others with some different hair? No, of course not. (There are exceptions, but pleaaase) Strategy games have their method of having the player decide which unit or building they want to upgrade and so support the guild their in. Some people in the guild provide defensive units, the other ones offensive, and by swapping some of them, the players not only get significant roles in a guild, but also a quicker and better way to get a full hardcore army for kicking some asses. That's just a basic level of customization. In fact, when one says "customization", the others don't have the image of building or skill trees, but putting a logo onto a spaceship or such. If your technology allows that, put it in, but for example being able to design your own spaceship out of different parts into a decent fighter can be much more fun and engaging. If you have such a significant feature, make sure the user learns it early, so one does not get bored until the point when he finally "gets to the awesome feature they write about everywhere". What a big nono for customization is, is a customizable UI. The UI should everywhere be consistent. Not only is there lot of development hell associated with it once you upgrade your game, but if somebody for example shows a screenshot of your game, you lose the possibility for fast recognition. You don't want somebody in a "Your Game" related forum to post "Woah, which game is this?". However, if somebody makes a Greasemonkey script or Stylish css style, make sure you track the popularity of the modification, so you can possibly add the most popular features to your own version of the UI.
This one should be the most obvious, but it's not. Often game developers create their games with no obvious introduction whatsoever. Most of the time it's just a lame link to a forum post where somebody states that you should do this, that, then that first and then you'll see. The user shouldn't even leave the game to get this information. He could maybe get a dramatic message with "we are crash landed on this site, let's make sure we have enough energy supplies", which also generates a quest. (More to that later) Players automatically engage the whole system and start to understand it. Thankfully, in the last years this point got more attention, so you can see fewer games implementing this cumbersome style. In the course of the game every new element to the gameplay should have a short explanation of what it does and how to successfully use it, but don't overwhelm the user with too much information. Sometimes even a label with a quick explanation and a link which throws a popup where you can get more information is sufficient, but introducing the gameplay while playing the game is much more immersive, and well.. addictive.
These points are just to make the game more addictive, but you can't have an addictive game without actually having good gameplay. Which game you will make is still your descision, but here are a few pointers which may help you play your game even better.
What feels good?
1. A living, breathing, changing world
In browsergames you have the possibility to create lots of environments without much graphics by just using text. You may be on a planet, and an indicator for "It's raining, grain generation+" can always make a player more happy. Some browsergames implement some sort of map, where every planet/building/field has a different color code to quickly distingish which type of environment we have here and what we can do with it.
Very clever developers may even implement a day/night cycle into their games, where at different daylight times there may be different events or even some new gameplay options (for example a black market for stolen equipment). The possibilities are endless, and the player gets a feeling that the world he plays in is real. If the NPCs don't only fly around, but if you say have a bad reputation with the pirates and they start attacking you often, you succeeded in making a living, breating world.
2. Analyze how people play, evolve around it
This is actually very hard to pull off. Say the players found some workaround on how to do a thing easier, and you as game developer don't want that sort of thing in your game. Stop. Make sure you discuss that with the players. Why did they do it the easy way? Was the original way not fun enough? Does this new way actually support game play? Also, say if people always go kill in an RPG in some zone where some weak monsters drop good items. You may modify the zone monsters to drop something less powerful, but you can place a hard to kill boss in there, who drops even better items. So people now not go kill alone these small little useless monsters anymore, but may switch to a new target, and for example even finally form groups.
Also, log as much as possible from the players. Don't invade their privacy, but check out which skills get used the least, which one more often, and then you are able to balance them out, if you please. Before any change though, make sure you talk with your already established player base, but don't be too soft on them, as you may do something that is good for the overall game.
Oh yes, the fun part. Todays game always seem to have some kind of goal tracking system. It's fairly easy to implement and absolutely supports the way how people approach that game. There may be very easy achievements for everyone, and some hard obtainable for hardcore gamers. These are as easily implemented as ranks, so the more ridiculous, the more fun you will have developing the game, and the more fun the players will have playing it. Some addition to this achievement system would be to have them to have different levels, so you may get a bronze badge if you kill 10 monsters, and can upgrade it to silver if you kill 100 monsters. Achievements should be easily viewable by everyone, so players may ask each other "wow, how did you get THAT?" and that is one form of social networking which is very pleasant to everybody.
4. Everything should just feel overpowered, but not actually be overpowered
This may sound strange, but it actually can also differentiate between a good game and a mediocre game. This is actually a point where each game has to find the perfect balance point for itself. For RPGs for example you can have a powerful skill in one minute, but in the next you are overwhelmed by a very strong enemy, so you upgrade your skill to take that enemy down. The curve has to be steady, but not too much.
5. Quests are good for you
Now this is just one part which everyone likes. Somehow people like to have goals and at the same time float around and do nothing. With a quest system you can have that freedom of doing nothing but still everything. They can even keep a player stuck to that game if his friends for example are all offline. Quests should have a nice descriptive text, but the quest message should always have a short explanation of the quest for quick reference as well as a peek at what he gets when he completes the quest, so the player is actually motivated to do the quest. The perfect quest is a quest chain, where the player has to find a larger group of players to get the last reward. A friend finder this instant! Sadly, I played some games, where you get only 5 or so quests when you start the game and that's it, only 3 more through the whole course of the game or so. It's kinda strange to see that, especially if the game revolves around doing the same thing over and over again. Quests can give that diversity the game probably needs. If one stays all the time in his base, a quest can move the player to attack an unknown zone, and so explore even more gameplay possibilities.
So that was my small article on how browsergames work and how one could improve them. I was very sleepy while writing it, so please excuse any errors you may encounter. I also hope to see more games to implement some of these features mentioned here, as that would help us to see games of even better quality.