You can unlock higher ranks and their corresponding bonuses by earning more points and getting higher a "difference" score.

Corporal: 30 points, 30 difference, can create squad option.
Sergeant: 100 point, 70 difference, can create clan option.
Sergeant 1st Class: 200 points, 100 difference, unlocks Shotgun and SMG.
Master Sergeant: 350 points, 150 difference, unlocks Medic badge.
Sergeant Major: 550 points, 250 difference, unlocks HGH and ISA LMG.
Lieutenant: 880 points, 250 difference, unlocks Engineer badge.
Captain: 1,100 points, 300 difference, unlocks VC9 missile launcher.
Major: 1,450 points, 350 difference, unlocks Tactician badge.
Colonel: 2,300 points, 450 difference, unlocks Saboteur badge.
General: 2,800 points, 500 difference, unlocks Scout badge.
Lieutenant-Colonel: 1,850 points, 400 difference, unlocks grenade launcher.