How knowledgeable and friendly the employees are is important to me. Also important is that they accept beginners and answer questions, no matter how stupid.
However, I am most loyal to shops that have chosen a charity to support (especially if it is one I am involved with!). My best experience with that was a shop where I once lived that gave our Project Linus chapter a place to meet, was a blanket drop-off location, stored some of our fabric for us, and the icing on the cake -- they even made up simple kits for us. A shop doesn't have to go that far, but they can do little things, such as announce work days in their newsletters, offer a door prize for a quilt guild retreat, offer a premium for a quilt show or fair quilt, display a Linus quilt in the store for a few weeks, or in the case of our local Bernina shop -- offer space for a Saturday work day for an Eagle Scout cool tie making project.
These kind of actions also promote sewing/quilting, and what goes around comes around!
Dayle