Copy-pasted from my Steam review.
So I got the chance to play L4D2 at Comic-Con, and since someone wanted a review here it is.
It's essentially the same as L4D. There are 2 new special zombies: Chargers, which are rather self-explanitory given their name, and Spitters which are by far the best (best in L4D standards meaning grotesque) zombies so far. Just Google "Spitter" and see how ugly she is. Yup, "SHE." L4D2 no longer discriminates against women being special zombies (not counting Witches) so now you can lawfully bludgeon females into submission. Anyway, Spitters spit acid at you which then covers and burns the area for a set amount of time (much like a fire would) and then they run away like the women they are. Spitters seem to spawn more often than Chargers and are fairly easy to take down. The only difficulty with them is that they are very thin (Google her already!) and duck a lot. Chargers are about 1/2 as hard as Tanks and their only difficulty is that when they hit you you get sent flying very, very far usually resulting in you being seperated from your teammates with one ornery Charger barreling towards you.
One small note, special zombies seem to spawn together more frequently. We only played one part of a campaign, but we often encountered 2-3 special zombies in each group. We only saw one witch which was politely sitting right where we needed to go. I've heard rumors that Witches can now walk around, but our witch remained stationary. Attempts to beat her with a bat proved futile as she automatically locked onto the worst player aka me.
Speaking of bludgeoning, melee weapons are introduced. There are a variety of them such as fireaxes, bats (which make the same sound and motion as the Scout's bat interestingly enough), and frying pans. While playing with a bunch of TF2 cosplayers we had fun watching RED Sniper wail on a Spitter with a frying pan for teh lulz. The only bad thing about melee weapons is that as soon as you switch back to your gun you drop the melee weapon.
Let's see, what else. Incendiary ammunition only give you one round (8 shells for the shotgun etc) but are very effective against everything.
The new characters are decent. I played the big black guy who's name I can't remember (Coach?), while 3 others took the reins of the others. The white male (I have no other way to descripe him and I'm too lazy to Google him) seems a lot like Scout meaning annoying, and I don't have much to say about the hispanic male. Black female is the new Zoey, although the special zombies seemed more intent on Mexican food than woman during our round if you get what I'm saying.
The level we played was called Dead Swamp I believe, and it was, well, a swamp. It was beautifully rendered and I got lost often. One part had us running along wooden boards in the middle of the muck because the swamp around you would slow you to a walk which I thought was a nece feature. We had a bridge break on two of us (leading to an amusing battle with a Charger), and our safe house was located in an abandoned warehouse of sorts, but otherwise looked the same.
Overall L4D2 was fun purely for the thrill of having new zombies. The weapons were ok, the map was lovely, yet the characters were not as personable as the classic Louis or Bill, but hopefully they'll improve when the final edition is released. Would I get this game? Depends on the price and if it'd work together with the original L4D (like downloadable content). As an entirely new game I wouldn't buy it. I'm not that die-hard, gung-ho with L4D so I'm fine with replaying Mercy Hospital to death.
Um, yeah that's about all I can think of. If I remember any totally awesome details I'll add them in the comments. Ask questions if you want too and I'll answer them as coherently as I can.