Don't like the art direction. Don't like the floaty honest-I'm-a-supersoldier-not-a-space-ballerina combat. Don't like they typical arena map design.
It's all fine, I suppose, I just never got it. And I don't hate it, I just rented Halo 4 and the campaign was fun. The MP was... marginally better because I never have to spawn with the AR again, if I play in the future.