Well what do you need on your HUD;
Health (or greying, blood on the screen)
Ammo count (only when firing)
Grenade count (See above)
Ability cool downs (see above)
Objective marker (constant)
Markers for nearby teammates
Most current FPS games fade out most HUD elements when they're not needed so it wouldn't surprise me if SD do the same, anyways that my thoughts.
Personally, i like to know how much ammo i have left in games like brink, where getting a resupply is going to be common, same with grenades. I would prefer to have a wealth of information and learn to ignore the stuff that doesn't concern me, than to have to keep mental tabs on the things that are going to fade away.