I tried it out the other day, and I have mixed feelings. I think I like it better than the original, but it leaves me with the feeling that most, if not all of what they did could have easily been done with DLC.
For one, it is certainly a lot more violent than the first game. I think it looks better, and shooting zombies is MUCH more satisfying now that they can be dismembered or have holes blown right through them. My favorite change is definetely the pipe bomb. When it exploded in the original, it just made a red cloud when the zombies exploded. In L4D2, it blasts the bodies and dismembered limbs through the air and they rain back down around you. It's actually kind of beautiful in a way.
I like some of the new weapons, the melee weapons were a great idea and they came out well. The defibrillator was a cool idea. The adrenaline is ok. The "boomer bile" bomb is fantastic.
The new crescendo events where the survivors have to go from point A to point B while being relentlessly attacked by the undead is a great change from the huddle-in-a-corner-until-they-go-away approach of the original.
I like that the witch is mobile before she is triggered now, but I can't help but feel they should have done the same for the tank. I'm not a huge fan of the new special infected, but maybe they'll grow on me. The charger just seems like a hunter that can't jump, but has more health; the jockey seems like a smoker that has no tongue so it has to jump on you like a hunter to pull you away; and the spitter seems like a long distance boomer that deals damage instead of summoning zombies and leaves a cloud of damage-dealing acid behind when it dies like the vision-obscuring smoke the smoker leaves behind when it dies. It just seems like a repackaging of the old special infected with new twists.
The new survivors are ok, but not as memorable as the original cast. The same goes for the setting. The original settings played out like some classic zombie movie scenarios; the hospital, the boat docks, the airport, and the abandoned farm house (my fav). L4D2 just takes place in New Orleans (as far as I know).
All in all, I'm still not convinced that it was a good idea to release L4D2 so soon after the release of the original. I still prefer the setting, characters, and infected of the original. L4D2 plays better than the original due to the improvements to gamplay and the various changes they made, but it doesn't "feel" better. To me, it seems like the bigger budget sequel of a great movie that looks and sounds a lot better and is more action-packed, but it loses the charm of the original.
I'll probably pick it up when it goes on sale through Steam.