After reading so much crap about the campaign, and not having any interest in the MP, I decided to wait until a friend was done playing it, and then borrowed it, so I didn't have to pay for it myself, and low and behold: you guy were wrong. This was so much better than COD4 IMO.
Played on Hardened, and I found the campaign entertaining all the way, just the right challenge without being too frustrating, lots of really memorable parts, with some making me say flat out WOW, even though I still have no idea of what was going on with the plot, but I don't care, I loved the fast pacing, with it changing at a regularly manner to a diferent styled scene in the other side of the globe just before some sequence starts dragging for too long.
No infinite respawning enemies either (wtf were you guys talking about?).
I really found the campaign in COD4 to be pretty forgettable except for the glorious Ghillie in the Mist mission (with its style being mimicked in several of the mw2's missions <3 <3), but really loved this one and will probably get myself a copy, since I want to replay some of these missions eventually. Good job infinity ward.
edit: this was probably a hard to read post, I was typing things as they came to my mind, but it's late over here and I don't feel like structuring it better right now.