Well, MW2 and Bad Company 2 were equally as fun in terms of what they had to offer. MW2 was very fast paced, and was arcade-like, with a very large range of guns and killstreak rewards. Bad Company 2 was much more involved with it's maps and vehicles, along with destruction physics, requiring much more use of tactics. I played both games a lot; I got to 4th prestige on MW2, and level 35 (out of 50) on BC2.
They were both great games and both had their highs and lows. MW2 was very easy to play, while also being very entertaining, but it's downsides were all on multiplayer. The maps and physics of it meant camping was re-defined in MW2. Also there was lots of spawn killing.
Bad Company 2 required much more focus and skill than MW2 did. The weapons (mainly sniper rifles) were harder to use (bullet drop, large recoil, slow reloading), you could change your class in-game when respawning, and a wider range of tactics had to be used on the larger maps. The feeling of reward when getting a kill was much greater than in MW2.
I am personally not buying MW3, not because I think it will turn out to be terrible (which I don't), but because Battlefield 3 looks like it will be a much better game for me. I think both games will turn out to be very decent shooters, and I will rent MW3 occasionally.
There