The Silent Cartographer, Assault on the Control Room, Delta Halo, Tsavo Highway, The Ark, The Covenant... Bungie has created some of the most amazing levels in the FPS genre. So, what the fuck happened? The level design is so bad in this game...
The gameplay design leaves a lot to be desired too. All the cool mechanics that were present in the Halo games (plasma to break shields + headshot, shooting a jackal in the hand to stun them + headshot, vehicle boarding, disabling vehicles with the EMP from the PP...) have all been replaced with some incredibly generic mechanics. Basically all the enemies are bullet sponges, and the only interesting thing you can do is going for headshots to do more damage. Dull.
And BTW, why the hell can't we skip the cutscenes? Bungie please...