Halo is not a bad game. Or a great game. It's an average, and extremely overrated, game.
Halo 2 was worse. I haven't played 3, so I can't comment, but from what I've heard it's still not that great.
Most Halo haters who actually dislike the franchise don't dislike the games. They dislike...well...the franchise. The huge media boom over it, the thousands upon thousands of slavering fanboys, all ready to grab a sniper rifle and camp in a safe spot (camping remains the absolutely most cowardly tactic in a FPS game, ever, no matter what justification you use, with the sole exception of guarding a mission objective. And no, the levels' only sniper rifle spot is not a mission objective.)
A game that gets perfect scores from absolutely everyone should be great in all areas. This means that you should be able to squeeze lots of hours of fun out of it without using the internet, unless the game exists specifically and only FOR the internet (Battlefield series, MMOs, etc.). This is not the case with Halo.
The single player campaigns of Halo and Halo 2 are short, boring, simplistic and overall pretty darn disappointing.
The multiplayer is great? Sure, whatever.
Someone else said it. Halo is to the Xbox what Goldeneye was to the nintendo 64. A rather average and boring game that, because it's pretty funny to play with friends, got the rank of supermythological god-game by fans who simply cannot even consider seeing the faults.
Only Goldeneye DID add something relatively new, hit spots on enemies, and the enemy reacting to them.
And of course, someone else explained how Bungie didn't even try with the single player in Halo 2 and 3...
So, if they deliberately don't give two flying fucks about their single player campaign, why in the name of hades is it such a great game series? Because it lets you run around and shoot people? Like every other game ever?
A lot of Halo haters hate Halo just because it's trendy to do so. A lot of them hate Halo because they don't own a 360, or so it would seem.
In reality, it's more of the opposite. Most 360 owners are also rabid Halo fanboys, giving the 360, in the eyes of anyone who doesn't care about that console, an air of "That one thing that keeps dying to piss you off that has halo".
I, personally, don't mind Halo. The first game had an okay single player. The second game had a much worse single player. The third game, from what I heard, didn't improve on single player a single, itty bit.
And as Yahtzee says, maybe the multiplayer excuses it, but it'd have to teleport whores into the room before I (and a lot of other people with me) start caring.
I don't hate the Halo franchise because it's popular. I hate it because frankly, there are much, MUCH better games out there, even in the FPS genre, that deserve infinitely more praise.
I see the Halo fanboys' reasoning though, because frankly, if your only PC is pre-pentium (and used only to yell at Halo-haters) and you have a 360, I'm sure Halo 2 and 3 seem like great, wonderful, innovative FPS games. Anyone who played videogames before the original Halo came out however knows very well (or should) that frankly, it's not as great a series as it's cracked up to be. I honestly believe that without Halo, first person shooters worldwide would be better, more innovative, and more fun to play. (I mean single player, of course. Because if a game HAS single player, that is what I will measure it by, with the multiplayer at most adding two points to an "out of ten" score.)