It's simply 'reactionary' hatred. People tend to hate anything that gets overly popular. There were people who hated Halo before it was even released because it was getting so much hype. The same goes for just about anything else that gets huge - you'll find groups of haters for things like Harry Potter, Twilight and Avatar; people that even refuse to watch these things because they already hate them. And it's not even 'hate' per se, it's usually always an exagerated way of people saying 'this just isn't for me' but they forget to be eloquent about it.
Like for me, personally, on an average day I would say I 'hate' Halo. But I don't really. I've played and enjoyed all of the games. The only problem I have with them is that they are really not as good as most people make them out to be. The first game had a cliched sci-fi plot, FPS gameplay that had been done a million times before and was way too short, with no replayability (when you have other games to move on to) and limited multiplayer. Yet there are legions of fans who claim it was innovative. Yes, it's graphics were good, but what do you expect from the launch game of what was the most powerful games console at the time? Also, there were comtemporary PC games that already looked as good or better. Yes, it was a good FPS on a console at a time when consoles were reknowned for having few FPS games, but it wasn't a breakthrough for that - Goldeneye and Red Faction, among others, had paved the way. When Halo 2 added online gaming to the series, fans started championing the multiplayer as the thing about Halo that stood above all other games, and yet for me Unreal Tournament 2004, which was released 8 months earlier, was everything that Halo 2's multiplayer was and more. UT2004 had more of everything to start with and was the kind of game where you could play a new map for free every time you played it, instead of having to pay for a few new ones every 6 months. The single player in Halo 2 was again short and the plot disappointing, but I enjoyed the gameplay, especially playing co-op with a friend, but I didn't touch the game after that.
In summation, I see why people like it, but it's just not my bag. Perhaps it's because when it comes to single player experiences I prefer the 100+ hours of plot than an RPG can give, and for multi-player I like games that are endlessly moddable like Unreal Tournament. I may say I 'hate' Halo but I'm just being flippant - I don't think it's a bad game, but I do think it's way overhyped. At least I gave it a fair chance though - I bet there are a lot of people out there who say they hate it but have never played it. I put that down to the reactionary hatred for things that happens when they reach an above average level of popularity. It's like another thread I commented on on here, where people were discussing which movies they 'refuse' to watch, and a lot of people were refusing to watch Avatar because they were sick of hearing people going on about it. Ever stop to think that everyone is going on about it because it's worth seeing? It's like that. Don't say you hate Halo because so many people go on about how brilliant it is - play it first, then say what you hate about it when you can back up your statement!