I think a lot of the hate is directed at the fans and the people who decalred halo 3 the greatest game ever...
Personally its the fans and the hype, the games are ok, kinda do the same thing a lot, but now cods done the same thing and released the same basic game with a few changes.
Halo was just kinda meh, but got boosted to the ranks of awesome for reasons i cant see, besides a lot of the hate faded away once they declared reach would be the last halo game (none of my friends believed me when i said halo wouldnt be allowed to rest in peace, but im a silent hill fan...), but now microsoft have predictably brought it back, itll probably get more hate when halo 4 comes out, unless they try something beyond spartans beating the silly out of the same horde of aliens theyve been doing it to for the last half dozen games or however many there are.