Left 4 Dead 2 is an awesome multiplayer experience and gives you more variety than the first installment.
I'm going to tell you the good aspects of the game first. I enjoy being able to select twenty four guns/melee weapons over the six that were featured in the first installment. Also, you can modify the ammo of the weaponry and you can later add some attachments to some of your weapons. The variety of weapon classes are a decent perk as well. For example, there are three shotguns, three rifles, etc.
Another good aspect is that the levels are longer and this means that it is tougher to survive. Ammunition seems to be more lightly spread around levels and the length is surprising. Also, the finales can be difficult. The first one was quite a tough challenge for me and my friends and it was different from some of the challenges you saw in L4D. For example, the first campaign requires you to fill up the gas tank of a car before you can escape. If you don't do it right or if you take your time, you're finished.
I also liked the introduction of melee weapons. It enhances the notion that you're trying to survive and you'll use any sort of weapon to do so, including a frying pan! Also, the introduction of a defibrillator is more realistic than finding that your partner has teleported into a closet after being ambushed by a group of undead.
The realism mode is well done as well. Your partners no longer glow, headshots are almost vital and there is no incapacitation. Once you die, it's over for you. Well, unless your friend has a defibrillator handy!
Also, the AI Director has grown intellectually. The tank will no longer beat on a downed survivor. He will move to a standing survivor after he's disposed of one. Also, zombies are more aware and hear unsilenced shots realistically. Plus, there are more of them. Well, when you're playing well.
Oh, and let's not forget how you can leave gash marks on zombies from an axe or blow a gaping hole in their chest from a short range shotgun blast. That was pleasing!
The bad things?
Melee weapons are *EXTREMELY* overpowered. I was able to quickly dispatch a thirty man horde with a few swipes of my Katana. Also, despite the fact that the storyline carries itself better, it basically contains no real storyline and single player replayability dies fast.
The dependency on other team mates still remains, which leaves plenty of room for the other player to screw you over on his/her whim. There is absolutely no option to free yourself and that seems largely unrealistic. For example, having a Jockey on your head, you cannot at least *ATTEMPT* to throw it off (Which would be realistically easy) you must have another team mate shoot it off.
Also, the spitter can be overpowered. On normal, seconds within the noxious acid means half of your health. A few more? Death, and I mean FAST. Also, ammunition is once again infinite. I was hoping to see the team quickly scramble for their melee weapons after being drained of their ammo and then proceeding to go on a zombie assault tour. This does not happen.
Also, my one major problem was it feels almost like a parallel experience besides the perks. Essentially, it feels like an expansion. The gaming mechanics are exacting and Valve's engine remains unchanged but it could be better. If L4D was a vanilla sundae, L4D2 is simply that same sundae with some whipped cream and chocolate sauce.
Overall, L4D2 is a great improvement over the first game and is quite entertaining to play. It's good that Valve decided to listen to their fans and it's a gratuitous effort.