I've yet to make a shoulder holster, but I've worn them a number of times, and can give you some input from my own experiences.
Wide straps, particularly where they go over the shoulders, are much more comfortable than narrow ones.
The more flexibility in the middle of the back, the more comfortable the rig will be.
For "horizontal" shoulder holsters, the more angle adjustment that's possible, the better. Also, for those, some sort of ammo carrier on the "off" side helps to balance the weight, especially if there are no belt tie-downs.