I hated 2 on PC, but loved it on xbox, so these are my thoughts on xbox. Also I'm only referring to MP.
I'm far more impressed with 3 than I thought I'd be. This is how 2 should have been. It's very, very similar but without a lot of the bullshit MW2 had. A lot of the silliness has been removed.
Regular MP: Same problem as all CoDs, too unrealistic. Guns are unbalanced and don't do enough damage. My only real complaint is that like MW2 and BO they force you to play regular before HC. But then again if you're good leveling is fast enough it's not a huge deal.
Hardcore: Still not as good as 4 imo, but by far the best one since it. In-balance problems are gone in HC as far as I've seen. Weapons do just enough damage to be realistic, ranges aren't absurd anymore. Shotguns finally are like they were in 4. The absurd ranges are gone, but they're still effective.
Only weapon complaint is sniper rifles. It's like they went the total opposite of BO and made quicksniping WAY too easy. Though it's not a huge issue imo because most the maps aren't overly conducive to sniping.