Not really any one thing, since each quilt is different. I work to try and get seams and intersections to match and to not chop off points, but like some others, I give it three tries to get it right and then let it go. I have been known to un-quilt an entire quilt if I decide I don't like the way the quilting is looking, and I just un-quilted 3 entire borders that I'd quilted feathers on before noticing a tension issue on the back. I spend a fair amount of time auditioning fabrics for color and value when making a top and also for binding once the quilt is done. Despite all the above, I'm definitely not a perfectionist as I fall squarely into the camp that says done is better than perfect. I've also found that over the years as my skills have improved, I've gotten more picky about some things and less picky about others.
Rob