Honestly, I felt it to be closer to an "A" type sequel than "B". Tons of things from L4D1 are there (such as recycled items, sound effects, textures, models) making the game feel more like an expansion pack than a new game. Some of the new weapons are damn near literal re-skins of the old ones (assault shotgun and sniper rifle come to mind) and they handle exactly like their L4D1 parents who also happen to be in the game as well. Subjectively, I didn't find the new characters to be nearly as entertaining as the first 4.
The new infected are great and really help to fill out the (previously) lacking enemy roster. They've definitely improved their map making skills both in terms of design and objective complexity. The director AI is reportedly better, but I admit I didn't have enough playing time to really see any differences. Being able to swap your secondary pistol out for a different sidearm or melee weapon is a nice touch too.
I didn't get a chance at versus mode, but I get the impression that if vs. is really your thing, this game is far superior to the first, if only for the extra special infected.
If you liked L4D1 and are just finding it to be a bit stale now, you'll probably like L4D2. I didn't find it to be worthy of a $50 price tag, though.
(I played the xbox360 version at a friend's house. Get the PC version if at all possible)