Threads like these are usually frowned upon, but if I had to say what people have issue with in Halo, it's one thing.
The game is so popular that it single-handedly gave the original Xbox life, and consequently, the console FPS genre. People hate it because it's popular, and it's cool to hate on what is popular without reason.
I would know; I used to do the same thing with Halo, and FPSes in general. "Hurr all you do is strafe around them and keep shooting. So not hard." I'm still on that "hate it because it's popular" phase with Call of Duty, but I at least try to have reasons there. Not all of them are good reasons, I'll admit.
(EDIT: Another potential reason is because Yahtzee says so.)