Because its different.
People complain about every game coming out, mass effect 3, skyrim, WoW expansions, you name it. BUT! Usually no one says that a cod game is bad on the forums, yeah they exist, but the majority thinks it will be the best yet. Want to know why? they change nothing.
This is why if you buy one call of duty the experience is the same as buying another one, this plays a big part in the hatred for call of duty, is it a bad game, no, is it a casual game, yes. (I understand that there are alot of faults with call of duty, but it is for casual players.)
But what would you like in halo 4? All of the “Haters” seem to want is halo 3 (or 2) With new maps, campaign, and better graphics. The point of Halo is not skill, its fun. This is adding a new way to have an arena shooter, every thing looks pretty balanced so far, (maybe the exception of the stability mod)
Halo 4 will offer a new, yet exciting game, Halo reach did this, it was not as good as halo 3, that is correct, it did have balance issues, but at the end of the day it was a good game, and more fun than cod or bf.
I do have a problem with unlocking the specializations via legendary edition, I am getting it, but it is not fair that before anyone gets 50 and with a little game play you get someone they wont get for awhile.