Another difference between "quilt shop quality" and others is the color saturation. If you look carefully at two very similar pieces of fabric and look at those color registration dots in the selvedge, you may see that there are more dots used, or a thin metallic embellishment may be used on one/missing on another.
You see this most often with a designer fabric that is popular but limited one year opens up in a different color way or other changes and becomes more wide-spread for the next year or two -- in addition to down right copies by other companies.
Now, I'm not particularly snobby and I'll use some pretty iffy fabrics for my "use" quilts, those that are just blankets. I recently did a top of all metallic fabrics, a couple of those were gotten from swaps and were not quilt shop quality... but in terms of being usable fabric they were just fine even if ugly compared to the others. Still, they were never going to make it into a top except that one so I put them in.
I used to do a lot of fabric swapping and I know I have wider tolerances than some people over what is acceptable. But it's hard to look at a stack of fabric and not see those that stick out, either in terms of quality or adherence to the swap theme.
There is a whole level of fabric considered "Craft" fabric -- a lot of what (not to pick on them) Walmart sells is in this category. Even if it is 100% cotton, it isn't really quilting suitable. But some of the fabrics Walmart carries are just fine if you go through them bolt by bolt (if your store still has bolts!). Last year I bought a large bag of scraps that was mostly Walmart, and some of the Wamsutta fabrics were lovely -- others not so much and some of what was in the bag didn't make it into my project because they were blend fabrics even though they looked very similar to the all cotton ones.