Biggest problem I have with it is the meh level design (it's neither interesting to look at or interesting to explore and doesn't have great flow) and the slow mid-range combat. Way too much wearing down of enemy shields for an assault rifle to be fun to use in single player. Combat is also a bit death-matchy in that it encourages very aggressive close-range tactics (or sniping)... I'm a mid-range combat guy. First look at the Halo was breath-taking but no real sense of beauty beyond that, just more of the same.
Reminds me Unreal which had the same sort of problems.
Reminds me Unreal which had the same sort of problems.