balanced quilting, visual impact, use of color and creativity. binding needs to be full to the edges. straight, squared, edges not rippled/wavy. borders proportionate to the center of the quilt. even stitches.
they see the visual impact of the quilt first just like we all do, then they start measuring and checking out the technique, and quality of workmanship. make sure you follow any rules such as size, hanging sleeve guidelines (if required) appliques need to be attached well (as with any embellishments)
the judges write up a little report on each quilt even if you do not win any ribbons their criticism is invaluable, read it and look over your quilts and learn, never take it personal...they are not making personal judgements, they do not even know who's quilt they are judging.