Makeup, attention to detail, and knowing how to pose.
If you don't wear makeup, your outfit buries you. Cosplay, in my opinion, is about the person even more than it is the costume.
Attention to detail, whether it's making sure everything is exactly right or adding on your own little details and designs. The most important details are actually neat sewing lines, clean hems, and a good fit to your body. (not too tight or loose)
Knowing how to pose makes a HUGE difference in both convention settings and photoshoots. If you stand meekly in a corner, people won't notice your nice work, you have to get out there and strike a pose and be confident about it. Even if you do something dumb, people don't read it as dumb so long as you at least pretend to be confident. Knowing where to put your hands and tilt your head and show teeth or not is a big part of it.
Those are the things I notice most, at least, when comparing a great costume vs a costume that blows me away.
Cosplays in Progress:
Haruno Sakura (Shippuden)
Theodora (Oz the Great and Powerful)
C. Viper (Street Fighter)
Twilight Princess/BRAWL Zelda (Green BRAWL ver)
Tharja (Fire Emblem Awakening)