What always impresses me is when people do stuff the difficult way. It just shows the love and devotion on a whole different level if someone for example makes an automail for their Edward Elric cosplay instead of just covering the hand with the gloves or crafts that pervy book for a Kakashi cosplay.
I'm also a big fan of doing stuff the authentic way if it's possible. Finding out how an actual kimono looks like before rushing to make something close enough. Doing a bit of research on the type of clothing of the time if it's a historical series.
A good makeup is also something I like, because it's my own weak-point. So I respect anyone that can do it really well.