* Come back regularly to play it?
* Pay to play it?!! :/
* Click on ads? (If the creator were evil, they'd confuse
the users into clicking on ads, making them think it's
part of the game).
As with everything, it depends on the implementation ;)
I think it's probably tougher to monetize a web game, but flash game creators seem to be doing moderately well with advertisements that run before you can play the game.
There's an interesting hook in what you're talking about: I could IM a friend and say "hey jump on here, let's play together". It's much more social than most web games out there which is intriguing.
In general most web pages feel like you're there by yourself. The model of seeing many other people there at the same time is novel enough right now that it should be a little easier to get people to tell each other about it.
everybodyedits.com is very close to this; anyone can create a level and then you can play while you watch everyone else play at the same time. He's got $500 in donations so far I think.
