I got to add this then: Repetetive and bland gameplay.
Just a handfull of FPS doesn't have bland gameplay, but all have their strong repetetiveness.
In multiplayer: It doesn't require skill as much as it requires luck. You need to hope your opponents aren't hunting for you and hope that you got yourself some good teammates. You almost never die because you sucked, mostly because you could'nt have your eyes everywhere.
Singleplayer: Cool story bro- wait... I haven't found any FPS that has made me want to know more of the story, except for Unreal 2. Then add extra blandness in the A.I and you got the campaign done!
That's why I don't like it.
But that's because I don't like FPS games and not just Halo. xP