The maps! The maps just aren't doing it for me. I've spoken to all my fellow halo fan friends, and they all agree with me; bring back some maps from halo 3.
Specifically: Guardian, the Pit, Sandtrap, Rats Nest, Foundry, Construct, and Last Resort! I understand that Valhalla was a fan favorite, and it's great you guys brought it back. But that's quite a tease if you ask me. If you're going to bring back a great map from previous games, bring back more than one.