All of the preceding posts have made excellent points and been right on the mark. I only have the following to add:
A hard use, exposed carry holster must protect and cover the rear sight of the weapon. When moving through buildings, it's all too easy to bang the rear sight of a weapon on a door jamb. When going prone, it's an invitation to shovel dirt behind the rear sight. The holster also needs to be resistant to the elements as well as abrasion and impact. Exposed carry holsters, especially for law enforcement, require thumb breaks, as well as the ability to retain the weapon without the thumb break secured.