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.

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.

what the hell happened? seems like you're having a terrible experience. sucks to hear that. 

Wow, sounds like you really had a disappointing experience with the game. That's defiantly never good when a game comes out with bad AI or installation bugs.

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.

I play both . I see them as the same game .

I was promised Chicago Ted! WHERE IS MY CHICAGO TED!??

I am disappoint  
 
Very Nice! 

Why would you ever play L4D with bots? That just seems pointless.

MustacheHero is disappoint!

I'm having a blast with the game. No Installation errors and No Crashes for me.

Oh man, here I am coming to the defense of L4D2.  Talk about your nightmare scenarios.  I have to say though, you're completely wrong about the AI having not been improved.  Bots in L4D2 will now explore to find healing items on their own, will use pills at half health instead of waiting until they're in the red, and their pathfinding is even improved considered the more complex level lay-outs.  I've even noticed an order-of-heirarchy of sorts with regards to the distribution of healing items, usually Rochelle will take first dibs for some reason I just can't fathom.  There's still situations where bots will stand 5 feet away looking in the other direction while a jockey tears your head off, but they happen less often than a bot in L4D1 would just stand there twitching right next to a player getting smoked to death. 

I think you people give bots the wrong reputation.  I'd rather have a team of bots at my side than most players I've experienced, you just have to learn their behavior patterns and adapt to their strengths and weaknesses.  They're more accurate than most players tend to be, they'll never pepper you with friendly fire, and they'll always be right next to you, which can be both a bad and good thing.  I could say with certainty that I was more competant than L4D's bots, but in a straight-up versus against a L4D2 bot I'd probably have to resort to cheap tatics that could out-wit its AI in order to win against it.

Dude, what? 
 
I've played hella l4d on both 360 and PC and the AI is retarded as shit.  
I would honestly be better off playing with 5 year olds. When they inevitably lost attention and fucked off, they'd still be more use than Louis...

" The game is great and surpassed my expectations. You need to play it properly. Get a game of versus going with 8 people with mics and you will realize the greatness that is Left 4 Dead. "

"I've even noticed an order-of-heirarchy of sorts with regards to the distribution of healing items, usually Rochelle will take first dibs for some reason I just can't fathom"