Honestly, to me the gameplay in the first and second was pretty much identical, aside from the addition of melee weapons. The first one, however, was much better in terms of characters, setting, and atmosphere. L4D2 did have a ton of great set pieces, though (I will always love that rock concert finale).
The second wasn't a bad game by any means, but the first one was, in my opinion, better in pretty much every way. Too bad the XBox multiplayer is all screwed up now. In online games, zombies are giant pink ERROR signs, and block your path when you kill them.
