If the L4D2 demo follows the same as L4D demo, it won't remain there forever, probably just until the game is released so grab it while you can. It's worth the download, it IS good fun, just lacks that "yeah, this is really cool" factor you got when L4D came out ... if L4D2 was a film, it would be Matrix Reloaded rather than Aliens
One game I'm thinking about is Dragon Age. I hear that despite the predictable (but still very good) story line, and it being a lot like other RPG games (which is no bad thing, why break from a working mold *cough* infinity ward take note *cough* ?) it's actually one of the best games of it's genre. I'm not really an RPG kind of player, but I did enjoy NWN2 so I may have to give it a looksee.