The HUD= The Map, Cheap gun arching position ie where to shoot your guns at. Your planes damage resignator (Where it shows what's seriously damaged etc.) and your Speed, Altitude etc. If you move the HUD down to minimal, it takes away the clutter off the screen, and you can be immersed into the game better. I have the HUD down to "none" on Realistic, as I don't like being told where to aim my guns when deflection shooting
Plus, when ever I tried aiming where the game said, it was usually wrong and wasted the ammo.
EDIT: I just use the gunsight, since I'm always flying in cockpit. Except for the German planes at the moment.