Just a quick note: this could be interesting: apparently, after all the bookmark, photo and video sites, there is one new „user generated content“ thing for which VCs put up their money: online games.
I doubt the potential is as big as for any of the other typical Web 2.0 sites, simply because it is so much more difficult to conceptualise a computer came, with all its rules, scenarios, player modes, levels, etc.
At the same time I am sure the market is still big enough for good advertising potential, since people will probably spend even more time on these online games portals than they will on Youtube.
(The only thing the sites need to make sure: that the players will see a range of ads, even though they are most likely spending a lot of time on a single page. Remember the discussion about the death of the page view?)