There are two reasons people dislike CoD: armchair game design with an unwarranted sense of elitism and aversion to anything popular with an unwarranted special snowflake attitude.
More OT: I'm not sure that I agree. I found MW2 multiplayer to be fairly hard to get into compared to a lot of shooters. Admittedly, this might be because I had more experience with more traditional shooters, so they were easier while I had little experience with the MW stype of FPS.
On the other hand, I really enjoyed MW2. I liked the campaign, despite everyone's griping - it was long enough to satisfy me, provided some neat set pieces, kept the action flowing at a decent pace, and varied the environments and situations pleasantly. And, while I struggled with it for a while, I really did enjoy the multiplayer.
People who complain about the fact that CoD is "just about whoever sees the other guy first" are both absolutely right and completely wrong. CoD is indeed primarily about whoever sees the other guy first. The issue is where people think that that's some sort of unintended design flaw or some kind of problem. It's not. CoD is just a different sort of shooter - it matters far more where you are, what sort of cover you have, how and when you move around the maps. It isn't so much that there's no way to win gunfights because they win if they see you first, it's that the way you win gunfights is by putting yourself in a position where you'll see them first. Personally, I much prefer this sort of tactical play to traditional FPS games where such issues confer only a minor advantage to the point that a player with especially good aim need not worry substantially about them and fighting smart can't ever really beat better aiming ability.
TL;DR: There are plenty of games that are decided more or less solely on reflexes and muscle memory. This isn't an inherent quality that all shooters must have. It can be nice to have a game that focuses more on other aspects of player skill too.
