Replay-ability is key to not only getting players into your game, but at being able to keep them in and get them coming back. Taking a look at the front page games you can see that almost all these games have a re-playable element to them.
Roleplay while it does not contain a set ‘story line’ or achievements to get players coming back, it utilizes the idea of being able to do really what you want, and as it suggestions, ‘Roleplay’ which younger audiences love and will generally come back, even if the game has no real moving forward aspect.
Simulators these games can be tough to really get a re-playable element in, as it’s generally just the same loop over and over again. But just the name itself seems to get players playing. Majority of these are 1 hit wonder, that pop up on the page, and don’t really offer much.
Then we have a few of the ‘other’ (not to degrade them, but couldn’t come up with a title that fits them all ) games like Phantom Forces, Murder Mystery 2, etc. which all contain some sort of levelling system, and upgrades and customisation. Players will want to come back and play more to get higher levels, to get the best items, to be the ‘best’.
Most importantly though is to actually listen to what the players have to say in your game. When you get people playing, ask what they like/dislike, what could be added, changed, anything to get player input. They are the ones who are playing your game, and if you cater towards them, then you’re more likely to get them to come back, or even better, come back with multiple other users.
EDIT One other thing is to actually get them playing the game when they first join. A lot of games have you sit in a lobby till the next round or whatever. A first time player ain’t gonna wait 2-3 minutes to play a game they might not even like. Get the player actually in a game, and actually doing something within 30-45 seconds of joining. They are less likely to leave if they actually get to play the game in that time span.