I enjoy quilts where the quilting complements the piecing (or applique). It should also complement the intent for the quilt. I was in a Melody Crust class last year where we brought in quilt tops to generate ideas to quilt them. So a quilt given as a wedding gift might merit more intricate designs and a quilt intended for a child to drag around and cuddle may be better off with a simple meander or straight lines.
Personally, I'm not wild about the allover mattress style - but there are quilts where it works quite well. I'm developing my skills, so am envious of some of the work I see at quilt shows. I love some of the custom, more densely quilted pieces.
There was a local show I went to a few weeks ago, where there were two quilts that did make me sad. In both cases the piecing was beautiful and creative, and it looked like little to no thought was put into choosing the thread color for the quilting as it really stood out and not in a complementary fashion. Kind of a quilt sneeze. In the case of those quilts, less would have been more, or a few shades darker with the thread and it would have blended better.
Great point by Dunster on the way a photo is angled to highlight or hide the quilting. That in itself takes some thought. I love my digital cameras as it allows me to see quickly if I've got that right or not.
Cheers, K