[Legionnaire]

I'll be interested in others' responses, but I'll start. I'll assume the holster is for concealed carry, and not duty:

What makes a safe holster, safe?
- fits the gun it is made for
- holds the gun securely, with or without a thumb break, yet permits easy draw
- holds position on your belt so the gun/holster doesn't move around, so you grip the gun the same way, every time
- completely covers the trigger guard
- has a sight channel to prevent sights from snagging and slowing draw/presentation

What makes a dangerous holster, dangerous? Inverse of the above ...
- doesn't fit the gun
- gun is insecure and flops around or (worse) falls out, or is too tight (maybe thumb break is too tight), making it difficult to draw easily
- moves around on your belt so the grip changes position
- doesn't cover the trigger, creating possibilities of ND--or someone else firing the gun--while in the holster, or introduction of foreign objects that snag the gun
- sights snag on holster preventing a smooth draw/presentation

In addition to the above, a "good" holster is made from quality materials, holds it shape, is comfortable to wear, has a stiff mouth so that your gun can be reholstered with one hand.

... for starters.