I enjoyed halo 4 almost as much as halo 3, I figure there's no where better to go for a good answer than here.
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I honestly can't point to anything specific, but I played 4 for maybe 5 or 6 hours tops. I just didn't feel engaged by it. I loved Halo 3, played the hell out of it. I even liked Reach (sorry), but I didn't feel engaged by 4. PS: I didn't like the abilities in Reach, but I still had fun with the multiplayer. The story got me because you knew what was going to happen in the end, but you still fought. Maybe it's just my personality.