Proportions
Symetry (balance)
Rhythm
Movement of the lines that shape and define
Materials utilized
Decoration (sometimes)
Aesthetics of the finished product where the combination of all of the above result in a work of art pleasing to the mind's eye, the hands and functions perfectly.
Several designs listed in the above posts qualify.
Going back to the Glock as ugly statement, it may be that the Glock design committee followed the Form follows Function school of thought and as it was designed as part of a Martial competition, it is what it is and there's nothing wrong with that, it simply does not allow my "eye" to willingly soak up the vision of the finished product, compare and catalog that end result as a piece of finely sculpted or shaped metal that alone could be called beautiful or pretty, as it were.
The Glock is Martially brutal looking. Austere, square, boxy, with few flowing lines. Very masculine. You can look at it and know that there were few, if any, Italians on the engineering design committee. It is also a reflection of the time frame the design reflects as well as the method of manufacture.
While "Pretty" is somewhat subjective... some things are timeless, or at least are appreciated over a period of time by more than one generation of man, for the "artful thought" that went into them.
While I do not find Glock's to be pretty, I've heard several females say that the 26 is ... "cute." I prefer the lines of the 34 or 17L to the 19 and the overall shape of the 17 compared to the 20/21/30.
None of them can hold a candle to, say, a 5" charcoal blue N-Frame w/ checkered target or Diamond grips or an ivory stocked case hardened blue, 4-5/8" SAA or a 4" barrelled, french grey Luger w/ light engraving and good straw color.
See?
Subjective.