Having only played the demo and the actual game for a day ( See here for explanation). I've got a limited opinion on the game, but have noticed some things that kind of shed some light on why the game was made in a year.
 
First thing I noticed in the game that was different: No legs. MY LEGS ARE GONE! I believe it was only in the PC version of L4D that had it, but in the first game you could look down and see your character's legs, like Halo 2 and 3. Now while you might not say that's necessarily a problem, but for a sequel having something like that taken out, it's just very disappointing.
 
Second thing is how many problems me and other people are having with installation issues and crashes to desktop.  Again, obviously not a problem for 360 players, but a big deal to me. Why would they release a game and not even have the installation issues fixed? Surely, they had installed the game on their own computers right?!
 
Last thing for now...AI. The AI was great in L4D they would help you up, strafe the tank, not startle the witch and actually pick up weapons and items when you wanted them to. With L4D2, well...they're just retarded. They can't do anything right! They choose melee weapons over any gun, leading to their deaths when trying to melee the tank and they don't pick up "better" weapons. They'd rather stick with their Silenced Uzis than pick up an AK-47. Lastly, they don't even pickup medkits. I was in a safe room and everyone had green health and a medkit, except for the one bot, Rochelle. There was an extra medkit and I press "Z" (AKA the "Look" command) so many times my Z key stopped working and that dang bot would not budge! In L4D, when you do that the bot comes running over to grab it! 
 
 
Valve, I am disappoint.