NRA editor Sherriff Jim Wilson has a great list of things you should consider when you shop for a holster for your defense.
In short the holster has to be,
1. made from quality materials
2. made to fit the specific model you carry
3. made to fit the belt you use
4. devoid of retention devices that impede your drawing
5. able to have its top open
6. available for weak-hand drawing.
From: Sherriff Jim Wilson
