1) Battlefield is just straight up better, which doesn't make me hate Call of Duty, it just makes me not care at all.
2) That whole thing with the original design team getting canned and hot swapped for a far worse team did make me pissed, mainly because people keep buying it, and companies shouldn't be rewarded for firing their talent and parading the license around like a fucking shameless, used-up puppet (which has happened a lot in the short history of gaming: Silent Hill, Final Fantasy, Resident Evil, and I'm sure a ton more I don't even know about). I liked the first Call of Duty, it was intense, and the developers are what made it so, and I heard the first Modern Warfare was good for the same reason but I can't really be bothered to check due to point 1. So yeah, this is where my ire begins to rise with the modern CoD franchise, and Activision for that matter.
3) People are constantly raving about it and bros are constantly playing it. I HATE when mediocre games are talked up, meanwhile I can picture a rather massive stack of all the original-epic-true-works-of-art games out there that the idiot foaming at the mouth for CoD has never played and had no interest in playing because original ideas are scary to such a person. Then I contemplate how this trend, for large hordes of people to buy mindless, mediocre-at-best, shooters, makes it impossible for the few really creative people we have left in the games industry to even pitch their game to a publisher because it's not going to make "Call of Duty Money".
So yeah, aside from the one logical, history based bit for point 2, and my simple taste bias (if you count better maps, player count, graphics, bullet physics, actually differing class types, vehicles, and destructible environments as a difference in taste and not just... you know... better... >_>), it really all boils down to one idea: "Fuck anything popular enough to make the big-wigs aim every game in that direction, and for that matter fuck the big-wigs."