3. Enemies Are Too Repetitive
Much like the missions themselves, the enemies aren't much to shout about, and hordes of dregs, knights, wizards and so on will be on the receiving end of most of your bullets throughout the game. The vast majority of the enemies are best distinguished by their colours above all else, and the only enemies who really stick firmly in the mind are the larger bosses, who are nevertheless just idiotically-programmed "bullet sponges" who have zero personality. We don't expect the game to introduce us to these baddies on a massively personal level, but would it have killed Bungie to vary things up a bit? When we're faced with taking down the hundredth wave of dregs, things start feeling rather tiresome and samey. We know they're capable of better, because they gave their enemies in Halo plenty of character, and splitting the bug hunt up with some actual set-pieces no doubt didn't hurt either.