1.Firefight:
Although this originated in ODST, my main memories lie in Halo Reach. Not only would it be nostalgic, but it's one of those game modes you can just keep on playing (Mostly Corvette). Messing around with the settings was fun, like using the bottomless clip beam rifle or running around with x300 shields.
2.No Shiny Stuff:
Alright, I don't mean scenery. What I'm trying to say is that one of the things I didn't like was how everything seemed new and shiny. If you were really in war being shot at by lasers and rubble blasting in your face, you would have faded and/or scratched armor (Again, Halo Reach did this well). It was cool at first, and I know War games is a simulation, but still.
3.Weapons:
Is everything dropped with an ordinance? I miss having to find the weapons around the map, not calling in an overpowered weapon every few kills.
4.ODST:
What happened to they guys/gals? I have to say, I was super excited seeing the return of ODST armor in Halo 4, but was still a little confused after seeing no signs of them in actual gameplay.