I know this post is old, but to me it was the effects and sound effects. Nothing sounded right and the explosions weren't beautiful. It was a very beautiful game but the explosions were severely lacking. I think making such drastic changes to very well established sounds like the Warthog engine or ghost boost was a step in the wrong direction. Also, there was something about the pacing of the game. It seemed like you ran through it while also spending way too much time in a level. The pacing was strange and the ending was lackluster and spectacular at the same time. I was hoping for a boss battle. What happened to Cortana was also a huge let down imo.