Ok so as I said, I ended up getting this game despite how much I absolutely HATED L4D and I've gotta say, I'm really like this a lot. It's really just a lot of little things, such as a lot of the dialog between the characters, that has really pulled me into it. For the past 5 hours or so, I've been playing through each of the single player campaigns on normal (because I'm terrible), and I expected to be bored by the beginning of the second one like with L4D, but instead I've played all the way through the first 3, and am starting the fourth one now. I really like how there's finally continuity between all the campaigns, as it really gives me a reason to keep on playing. I'm not gonna go ahead right now and talk about the various things I liked in each of the campaigns since a lot of the little surprises are what's making this first playthrough a lot of fun. The second campaign was definitely my favorite of the first 3, and playing it in VS will be a ton of fun. It took valve a year but they finally got this game right.
Oh and I said this in IRC but yeah any shotgun+chainsaw is pretty much exactly like playing pyro in TF2 which I thought was rather comical.