The bottom line here is awarding MW2 honors because it sold well, is like awarding your popular supermarket romance novel literary honors because it sold well. Sure, they are both popular, and there must be something good about them to a large group of people, but that doesnt mean they stand out as towering achievements.
That being said, MW2 is a fun game. I play it. I was playing it not 15 minutes ago before coming to check for interesting posts here... but I could also spend all day pointing out major flaws. There is a lot wrong with the game, and there is plenty of room to have legitimate preference for other FPS titles.
I do not really consider MW2 a "serious" FPS. Its a casual FPS , with easymode aiming assist, lots of gimmicks, and lots of instant gratification with lvling up and unlocks and etc. This is why it is popular. That does not make it bad, and heck thats why I like it on PS3. I suck at FPS with a controller. But it is very easy to see why a more console oriented, heavy FPS player would think it was crap.
MW2 definitely provided its money worth, and will probably continue to do so for me for at least another couple of weeks. Its a good game, it was worth the $$, and I will continue playing it and having fun. But I sure would not have picked it as any GOTY or suggested it was superior to deeper, more serious FPS games like KZ2.