I believe that the following changes would improve my enjoyment of this game:
1. Allow mouseover information on objects that are out of line of sight, instead of just "out of sight". It is obvious to experienced players what most things are most of the time, but learning the object sprites should not be required to play optimally (if you don't mind taking the time to examine things). This would also help when invincible/berserk/enviropacked.
2. Either remove full reload, or give me a reason to use it (eg. faster reloading). Currently, it is a death trap for the lazy, unwary, or inexperienced, and has no tactical value that I can see. Not only does it reduce your reaction time, it also takes longer that reloading one by one. (Actually, almost none; it might be able to be used to extend action-based timed effects such as Invulnerability over a longer time.)
3. Make invulnerability globes immune to damage on the floor. I can deal with health globes etc. getting destroyed by explosions, but this seems weird.
EDIT: maybe not. Also, this point is out of the theme set by the others, so feel free to ignore it.
4. Allow higher tier badges to substitute in for lower ones for skill rank requirements. I had 2 gold badges and 4 or silver badges before I got the 9 bronze badges required for Sergeant rank.
5. (not 100% sure about the mechanics of this one...) Change the way manual aiming works. Currently, it is a bad idea to aim directly at an enemy, unless you are using Gun Kata. This is because (at least some) enemies can dodge, causing you to shoot at a spot without an enemy on it, and this can be avoided by shooting behind the enemy instead.
6. +1 to the suggestions that Juggler not treat pressing a hotkey differently from equipping normally, enhanced AoE displays for explosives and shotguns, more visible rocket launcher sprite, and more easily accessible level feeling.
EDIT: 7. Change knockback direction calculations, so that enemies are always knocked along the path of the shot, especially if you are aiming straight at them. This would prevent one shot of a Storm Bolter Pistol from pushing an enemy out of the line of fire of the second shot (or first shot of a GCB making the next 5 miss...)