Okay, a point of view from a woman who never worked on her own cars . . . I like a shop to look like a retail operation, not a garage. Clean, well-lit, painted an attractive color, and without partially disassembled pieces of large equipment blocking the floor space. Cardboard boxes, unless just arrived, belong in some kind of non-public storage space. By the same token, a clean, organized and professional-appearing service area inspired confidence.
Although I understand the tremendous costs involved, it's much more reassuring if a shop keeps a reasonable inventory. If the whole shop has only three dry suits and two BCs in it, it does two things -- leaves me without hands-on data, and makes the shop look shaky in terms of its survival. If you don't have much inventory, try to arrange it so that it looks like more.
I appreciate knowledgeable salespeople and a low-pressure approach (both of the shops I patronize are wonderful that way). It's important for a shop to realize that I may come in and talk about something several times before I actually make a decision and buy it.
I also appreciate a shop where the path I have to take with my tanks from my car to the fill station is as easy as possible -- please, not up stairs, through a door, across the shop, out a door, and down more stairs! And I have to admit that I secretly love having the shop folks help me carry tanks when they can. I can do it, but they're younger than I am
But in the end, it comes down to customer service. If the shop is friendly, knowledgeable, and willing to go out of the way to get me what I want, I'm coming back!