I'm one of those people who's gonna pick up both this and BF3. But the one thing that's always bothered me about CoD is that it's the only game that discourages teamwork, where BC2 heavily encouraged it.
BC2 - "Use spot to take out an enemy."
Black Ops - "Jump off a cliff to gain XP! Kill yourself a bunch of times to give the enemy a sentry gun and get your Ghost perk!"
Of course you could always throw your sentry gun across the map, but then you have to hope that not only doesn't it kill anyone else, but that you're the one who takes it out. What an incredibly fun and totally not-asinine challenge. :
But I think Activision knows they're playing catch up at this point, and their 10+ year old Quake 3 engine isn't going to cut it next time. Which I really can't wait to see gone, because the videos I have of how unbelievably broken the hitboxes are in Black Ops make it the MvC2 of first person shooters, ugh.