What are the details that make a good IWB holster comfortable to wear?
In my experience the holster must fit the gun well. If the gun is too loose or too tight in the holster it won't ride right on your belt. I vastly prefer the holsters that are constructed like the one in the previous post made by UBG. The "wings" spread out the weight of the gun and make it easier to carry. A well made, stiff, belt is IMHO also a must. That kind of belt will keep the gun from sagging, will also spread out the weight of the gun and make for a more comfortable carry. All things being equal the lighter the gun the more comfortable carry. My favorite carry gun is a S&W 1911Sc carried in a ComTac holster. The Sc is a very light, very reliable, very accurate 1911 and is an excellent carry choice. FWIW