Great question - I'm often asked that and it helps me to focus on what I really go for...
I go for feel of the fabic - If I make a quilt that doesn't feel good, it's not going to be used or enjoyed. Is the fabric soft or stiff? does it feel substantial or wimpy? if it's a print, how do the edges of the parts of the design look? (If design looks like it should be precise but colors are not where they are supposed to be it's an immediate reject) If I fold or drape it does it have smooth drapes in the fabric? (If not, there may be a lot of sizing in it to make it look nice but which will wash out and change the texture later.)
I also look at how transparent the fabric is - can I see shadows thru the fabric? if so, it may be too thin for my taste in quilt fabric...might be fine for other use.
Is there a difference in the feel between the front and the back? in good quilt fabric there is usually no difference and that means I can use either side I want to!!!