What makes a successful browser game?

There are more and more browser games built to the market each day. Some of them becomes successful one, while some of them do not and disappear into dust. Actually what makes some of them successful and distinguish with the others?

There are several criteria to determine whether a browser game can be successful, namely:

1. Overall game idea
2. Graphics and game play
3. Game community
4. Ability to make a player come back at least twice each day
5. The way to make profit from the game

The first 2: overall game idea, graphics and game play are just too obvious.
A successful game must have a great idea. For the vast amount of browser games in the market, some of them are quite similar. There are large amount of Travian clones or OGame clones out there. To be a successful one, there must be some new and amazing idea to make itself a different and make the players think that they are playing a new game.
Of course, for the technology nowaday, the graphic is very important. No one will like to stay on with the game any longer if the graphic is poor and cannot convey enough of the game idea and feel.
Surely game play is a very important criteria too. Whether the game is easy to learn, the player is easy to grow, whether the interaction with the game world is smooth, inter-player interaction is interesting...etc. is very important for people to like the game. Game balance is the most important too. Whether in the game world, the weak one can ever be catch up with the strong players; whether new player have a chance to survive when all the experienced players had established their character/base. This is always one of the hardest part to maintain in a successful online game.
In the next few post, I will talk one by one about the other ingredient for a successful browser game, which are actually equally important, if not more, as game idea, graphic and game play.