I'm not a fan, but I wouldn't say I hate Halo.
I do hate how many people prop it up as the most innovative and original FPS of all time, which really just betrays a complete ignorance of the entire PC FPS scene.
And I do hate how many games ape the basic gameplay mechanics (recharging health, limited weapon slots). I've enjoyed such things in games, but these mechanics have a very marked effect on the game being played. Limited weapon slots tends to encourage really conservative weapon options (part of the fun of unique weapons is playing around with them in different circumstances, which is a luxury you can ill-afford when it takes up half your weapon slots), while recharging health can easily drain every last bit of tension out of a game if it's too easy to hide behind a wall until you feel better.
My criticism of Halo: CE really comes down to two things. Level design that doesn't rise above okay (confusing lay-outs further complicated by a lack of variety in wall textures) and fairly slow combat (I personally love a game filled with one-shot/one-kill cannon fodder enemies, which makes the combat feel much faster paced). I eventually just got bored with the game and gave up even before the oft-criticized Library level. These two criticisms don't make Halo a bad game, merely one that isn't too my taste.
But I think if you cut to the chase, I think people don't so much Halo as they hate the more moronic elements of Halo fandom. Having a very loud, ignorant, vocal group going around dismissing a lot of games that were far more innovative and original than Halo creates a certain amount of misplaced annoyance that gets directed at the game.