I'm a huge fan of the Elementary icon theme, Albatross GTK+ theme, and Xfce for many of these reasons. The themes are simple and easy-to-recognize but quite beautiful as well.
Xfce is awesome because of its configurablity and speed. Everything down to how your clock displays time is easily customizible (and 99.9% of time you don't have to edit a text file, though there is always that option). Most of all though, it is designed to be fast and to stay out of your way. There are no animations as there is no reason to hide slowness. Windows can simply pop-up instead of ssssllllliiiiddding into view.
User interfaces should not just be pretty, they should be elegant and even graceful, which is a much harder thing to achieve.