Not all inclusive, and in no particular order:
1) FULL firing grip, before gun even starts to move.
2) FULL sight channel. Galco has more than one where the sight channel STOPS 1/2 way up !
3) COMFORTABLE. If it ain't comfortable, you ain't gonna wear it ! ( a drop shank is NOT required to make a holster comfortable for a woman.)
4) SECURE. The gun MUST be there when you reach for it !
5) STABLE The HOLSTER must be in the same place everytime.
6) Trigger must be covered.
7) MUST allow one hand reholstering. This means the holster cannot collapse when the gun is drawn.
8) CONCEALABLE. While Duty rig and Outdoor/hunting designs do not need to take this into consideration, the rest should.
9) DETAILS. Did the maker pay attention to the details ? Dye evenly applied, stitches straight? Are the kydex edges sharp and rough, or buffed smooth ? If there's a "speed cut," will the gun FIT THRU it ? ( I'm amazed how many DON'T!)
10) BELT. You must have a GOOD belt to carry the load. You can have the absolute best holster in the world, but if you put it on the belt that came free w/ the pants, then you'll NEVER know how good the holster is.
There are more, of course, but these are the main ones.
Innovative Kydex Tactical Gear