A holster must permit a full firing grip to be attained on the pistol as it sits in the holster (trigger finger straight, natch). In particular, the holster must permit one's middle (driving?) finger to come into contact with the bottom of the trigger guard (for auto's) or the deepest postion of the recess created by the frame or stocks, behing the trigger guard (in revolvers). "Sweat guards" tend to work against this and I do not like them.
The trigger guard should be sufficiently covered to keep one's finger off the trigger while the piece is at rest.
The holster should have retentive qualities as required for its mission. Friction or molding of the holster body may suffice, or some sort of mechanical latch might be needed. An open-carry police holster should probably have a stronger and more proprietary retention mechanism than is needed on a concealment holster.
Pretty generic features...but it'll do for a start.