Jase, I'd put the campaign length at maybe a bit longer than COD 4, yes, it could be longer, but they've obviously gone for quality and not quantity. However, the levels are more open, so there are several ways to complete the objectives from what I've seen which would increase the re playability. I played the game on the second hardest difficulty (I forget what that is) and it took me a little over 8 hours, though I have seen people on forums saying they've completed the games in 5-6 hours!

, (but they dont mention what difficulty)
There is a new mode called "Special Ops" which are basically short missions that are designed for co-op play, you can however play them single player, but there are some that do require 2 players. Unfortunately to play this in co-op you do need a friend on steam
I just had a brief online game, as you may have read that there are no dedicated servers, only peer-to-peer (so just like console's), the majority of people there had a good connection, with only a few which seemed dodgy. I will check it out a bit more later on and will let you know how it is regarding perks, maps etc. But it all seems to be very stable which must be rare for a game straight after release.
You do need steam to install and play, even for singleplayer.
PC I'm running it on is
Intel E7500 Core 2 Duo 2.93GHZ (stock speed)
512mb 8500gt
2gb RAM
I'll post more details on MP and other things hopefully tomorrow sometime once I've had a decent go of it.
Aaron