I think the main reason MW3 is facing so much scrutiny is because the CoD series has run for so long and hasn't changed much. The series was once praised for the way it flowed, trying to keep the characters as human as possible. In Black Ops, the focus seemed more like watching a series of explosions, both on-screen, and plot related, nothing flowed, the characters were all gritty and dark, almost completely unlikable, and for a game that called itself "Black Ops" it didn't seem to have as much subtlety, it was almost schizophrenic in it's action sequences. MW3 seems to have only gotten worse, with, scripted explosions that are supposed to convey the truth that this is the s**t hitting the fan, but comes off as trite action, just another explosion in a screen filled with them.
Overall, MW3 doesn't stand out much, the series is spreading itself over to many fronts like the Nazis you defeated in the first game did back in 1945, and, in the same way, has suffered for it's lack of real continuity.
On a less spoken note, how many main protagonists have there been in the MW series? And I wonder if the main character will meet the same fate that his predecessor in MW2 met.