Well, some people simply don't like MW2, and that's fine.
But there are some people who think that if they say they hate popular things, it means that they are intelligent and sophisticated. This is a logical fallacy - sometimes popular things are popular for a good reason, rather than a bad reason. Bach and Mozart were both popular in their day, you know.
And then there are some people who like jumping on bandwagons.
Overall, while there are reasons for people to dislike MW2, I'm quite sure a few of the people who rant against that game haven't even played it, and hate it on principle, which is a silly thing to do. Somehow they have got it in their heads that only idiots play FPS games, as if playing RPGs or JRPGs or Strategy games meant that they were somehow "intelligent". Video-games are Video-games, let's not delude ourselves here - a grand intellect is NOT required to play almost any video game, and playing a certain genre or avoiding a certain genre says nothing about your intelligence at all.
Yes this is what I'm saying - saying you hate MW2 doesn't give you permission to put on a monocle, grab a glass of brandy and swirl it one hand while looking down your nose at everyone else. Saying you hate MW2 doesn't prove anything, other than the fact that you dislike the game. It doesn't make you any more intelligent than anyone else or any more refined in taste.
Of course, you can legitimately hate MW2 if it's not your type of game. I didn't like MW2's storyline or single player game very much at all, and after putting in about 50 hours into the Multiplayer, I stopped playing for good. MW2, in my opinion, is a decent game, but not gaming perfection.