Kill zombies, check. Repeat from L4D, check.
The only reason i bought this was because i borrowed L4D from a neighbor 2 weeks prior to L4D2 launch. No point in buying the old one since i figured everyone would be playing #2 online. Other than that, this game is the same as the first one with slight differences. The new characters are fun (Coach is my #1 favorite to play with).
Pros:
New weapons (guitar FTW)
New baddies
Zombie killing fun
Online multiplayer is exciting (scavenge especially)
Cons:
Some sections are ridiculously hard on the highest difficulty
Couldve had some different aspects of the game changed form L4D
All in all, i would recommend it to anyone who DOES NOT have L4D. Otherwise, it's not worth the purchase. Anyone who says any type of GOTY for 2009 is dumb. it had the potential to be really good but it lacked a definitive change from L4D.