There are many, many variations of holsters out there. Some are designed for a specific purpose others are intended to serve a wide variety of needs. Of all of the above what are the features that most if not all holsters should have (in your opinion of course) to qualify them as a quality rig. Also list why you feel such features are necessary. You do not have to list every possible thing; pick one or two and we will see where the topic leads.
Additionally I want to hear form makers (Kydex, leather, nylon or what ever) and users. Such an exchange will reveal why some makers do what they do and also provide good input from those who use the gear.
Last: let's keep this to an exchange of information without "commercials".
One of the features that I think is paramount in a quality holster is "Retention". A good rig must hang on to the weapon. The flip side to this is the "Break" (the point at which the holster lets go of the weapon). A clean break with no drag. IMO the best way to achieve this is with a form fitted Kydex holster.