A uniqueness. They are something, or have quality about them that is rare or not what you would expect. They do not fit any standard moulds and are not conventional in thought or mindset.
Many people you meet seem like fascinatingly unique people -- like a big scoop of triple chocolate Rocky Road ice cream with all the nuts and marshmallows -- but after getting to know them only a little bit, you see that all the uniqueness they are showing off is only a thin layer of Rocky Road topping....and its just predictable, standard Vanilla all the way down to the bottom. They are really very conventional people. It's so very disappointing.
Another thing I find beautiful is speed. Especially speed of thought and speed of comprehension. People who can grasp a concept immediately, or can form complex argumentation in only seconds...or can even come up with witty responses at lightning speed are beautiful to watch.
Talent. That's another point. Very talented people often have a beautiful quality to them (if they aren't bastards with it).