Fire the Secondary Guns on the Wraith
Why can't you fire the secondary weapons on the Wraith? The left trigger should fire the secondary guns, but instead it gives the Wraith a speed boost. It's a tank -- it doesn't need to be fast. Wouldn't you rather be able to fry some jacker who gets too close than have an afterburner? The NPCs can fire the guns and you can't. What gives?
Another possible use for the D-pad would be a lean function. Hitting left or right on the D-pad would let you peek and/or shoot around corners, while still maintaining partial cover. We fully expected to see this in "Halo 2" and were surprised when it wasn't there. Let's hope it makes it into "Halo 3."
Carry Mobile Turrets
So what's better than watching Johnson lugging that turret, setting it up and cutting loose on the Covenant in Cairo Station? Well...lots of things, but the point is that the Chief should be able to lug turrets around. It would work like dual-wielding. You press "X" to pick up the turret. When you're carrying the turret, you move slower and can't fire, but you can press "Y" to discard the turret at any time. You pull the right trigger to set up the turret. Once it's set up, you operate the turret in the standard way. There are several places in "Halo 2" where it would be great to lug a turret to a nifty vantage point and then cut some fools in half.
Now, on to environments.