I'll say what I said about Left 4 Dead 2. It's what a true sequel should be. It improves on the original and makes me wanting more. I'm starting my 2nd playthru (on hard this time) this afternoon and I know it will be a different experience from the first time.
My only beef would be the freezing issues in the multiplayer modes. Although on tuesday and wednesday I didn't experience any freezes.
DLC for MP is supposed to be out in March and more SP content after that.