I blame all the overfishing. We need to ease up, and allow schools to repopulate and for fish farms to rejuvenate the world's stocks.
... Oh, THAT cod. Whoops.
I've played very little CoD, so I'll throw that out there first. I've played Modern Warfare and Modern Warfare 2. The former was interesting, though it felt... loose? The controls didn't seem to respond as well as I'd expect (admittedly, the only FPSs I'd played till then were twitch shooters, with the accuracy ramped up to caffeine-junkie levels, so that may just be me). Modern Warfare 2 looked, and felt, almost EXACTLY the same. The only difference, as far as I can remember, was the fact that I played the former on the PC and the latter on the PS3. Apart from that, it looked the same and played the same. As OP said, it's not any less fun, but it's like it's exactly the same game (multiplayer-wise, anyway).
Just looking at the fact that they're releasing Modern Warfare 3 makes me wonder just how much innovation is being put into the series, and guessing not that much. After all, Modern Warfare 2 and CoDBlOps (wow, camelcasing!) sold extraordinarily well, so they're probably thinking that if it ain't broke, don't fix it. When they stop selling well, they'll try innovating again. When THAT'LL be is anyone's guess.