I think the armor permutations in Halo: Reach were the best espically the robotic arm. I really wish they would put that in this game. Also if you could use the same spartan in multiplayer for each game.
Please 343 add:
More armor permutations like Halo: Reach
Robotic arm from Halo: Reach
Same customized spartan from game to game in multiplayer
Also I think the ranking system from Halo 3 from the second title update where you had a rank for your exp in each playlist was really stupid and should be gone. I really liked seeing my actual rank next to my name instead of the white one for each playlist.