I can tell you I'm really enjoying L4D2. I liked L4D1 but wasn't going to buy 2 figured same same why bother, but friend talked me into it and wow it's twice the game as the first.
Longer campaigns, better graphics in daylight + weather effects, far more diverse and open landscapes, tons of new weapons, additional special infected, and maps designed so that endings don't consist of four players turtling in a closet for ten minutes.
They have new gameplay modes too, altough I've not yet tried scavenger.