It is different from L4D1. I absolutely loathe tards who say otherwise. "It's what L4D1 should have been." Yeah, and GTA IV is what GTA III should have been. Give me a break. It has 5 completely new campaigns, that isn't counting The Passing which they added on (because it sucks, just like Crash Course for L4D1, just not as bad). In every campaign in L4D1, you held out somewhere, whether it be a house or a rooftop. In L4D2, you wait, or you have to run across a bridge, or you have to fill a generator/car to get to the end. Finales are varied, which is nice. In L4D1, you had pistols only for secondary. In L4D2, you have melee weapons that allow you unlimited melee action, which was taken away in L4D1 to prevent corner camping and melee spam (which barely helped, just annoyed you at the horde). If you don't want melee, you have dual pistols- or you have a Magnum. The melee weapons range from a frying pan to an axe, and they all seem to differ a bit in the way they work (range and such). In addition to the secondary weapons, there is also a nice variety of new primary weapons including a Spas-12, AK-47, semi-auto sniper rifle, etc. These are great, considering L4D1 didn't give you near as interesting weapon choice. Instead of just pipe bombs and molotovs, you have Boomer Bile added into the mix, which can be a godsend at times. To add to that, you have shots which allow you to heal faster, run faster, revive faster, etc. Every add on was something that had to be thought up, and you don't just patch these things into a previous game. Valve came up with 4 new characters, and they all represent the area where we play, just as they did in L4D1. Hell, you even meet up with the other survivors in The Passing, which is very interesting (poor Bill...). And to boot, along with the new modes, they are having plenty of mutations that make the game more interesting.
All the content they have offered warrants a new game, and if you don't see that, that's your problem. Valve has absolutely spoiled the gaming community with TF2, offering class updates and maps and such, because that all FIT in the game, and TF2 is something that you continue on like that. L4D isn't your standard multiplayer deathmatch FPS. I for one, am ready for L4D3. Bring it on.