So I've played L4D2 for awhile now. It's fun but I have to agree with some of the complaints. I'll probably continue to play it but I'm actually not sure its really superior to the original. I actually like the new gore system and feel the shotgun to autos are a bit more balanced. The spitter is a welcome addition for forcing the survivors to move, which was an issue in the first game.
After actually playing though I kind of feel like L4D1 to L4D2 is the same as RtCW to Wolf:ET. A lot of stuff is bolted on that while kind of cool, doesn't really fit with the existing elements. It either isn't compatible with a core gameplay mechanic or is redundant.
The weapons:
I guess it's nice that there are different versions of shotguns and autos, but I would say the differences are minor enough that the weapons don't really add anything. How is the bile bomb really any different than the pipe bomb? If they were going to add another grenade I'd prefer it actually be something other than a crappy version of an existing one.
Melee weapons, in addition to being over the top cheeseball seem to cause a lot of friendly fire incidents when you're firing on zombies and your teammate rolls in from off screen in front of your crosshairs to chop up zombies with a chainsaw. Yes, I realize that was a bad play by a teammate and by me but still.
I haven't used the big machine gun or the grenade launcher so I can't comment much there...except that the grenade launcher seems better at doing damage to the survivors than at effectively taking down hordes.
Items:
Adrenaline doesn't seem a ton different from pills. I do use it to escape at the end when my health is low so I guess its useful but it feels a little redundant.
I don't know what I think about the defib. As it stands I always take a health pack unless there isn't one instead. I guess if there weren't survivor closets it would make more sense.
Explosive rounds and fire rounds? In addition to not making a lot of sense I find these kind of clumsy
Special Infected:
As mentioned I like the spitter. I'm not sure the others add anything though. It seems like the charger serves a similar purpose to the smoker and maybe the smoker should have just been buffed instead. The jockey seems like a slightly different hunter.
Maps:
Mixed bag for me. I liked Dead Center but Dark Carnival is just goofy with all the running on tracks and through merry go rounds...as if the survivors couldn't just climb through the fences. I assume Cold Stream is the one with the river...I can't say I really cared for the water pushing me around but I haven't played that one with a good team yet. Swamp Fever is OK, but I didn't like the outdoor map much in the original either. The Hard Rain mechanic of stabbing a fuel tank on your back sort of made me laugh...why can't I do that with regular fuel tanks?
The fuel tank grabbing finale levels are a little odd, they're kind of fun though. I know the game isn't realistic exactly...but I have a hard time getting over that it takes 400 gallons of fuel to START a car or a generator. And why are their fuel cans all over a shopping mall?
My main complaint about the maps is they're all kind of bright. I preferred the darker theme of L4D.
Survivors:
Just aren't as charismatic or likable I guess. Not huge. They go with the brighter more over the top theme they were going for so they do fit.