Sadly, I suspect that the majority of issues being experienced here are attributable to Steam not being able to handle the massive demand. Remember, this was the 1st weekend since release and I think massive amounts of people tried to play and the servers simply could not handle the load.
I can understand using steam as a digital distributor, but FFS, stop using them to authenticate games etc. It is so stupid. If Steam is down, then we can't play. I don't understand Activisions' reasoning behind this. It's almost as if they are trying desperately to show us how little they care about their customers. I would've said fans, but other than Bobby's mother, I don't think they have anymore fans, only customers. Look at the fiasco that was MW2. And now this. Yeah, it might be the biggest selling franchise of all time and blah blah blah, but that's only because of the customers and sooner or later they are gonna wake up and realise that they are being shafted time after time.
As for struggling with the SP, sorry can't help you there. Only issue I had was Steams' insistence on trying to download the game, despite me having the fucking disc in my PC!!! Again, Steam being a pain in the arse. EVentually closed Steam, closed steamwatch and retried installing and it worked fine.