"how good the mapping is" is pretty vague, i agree. but for me, the main important aspect of what makes a very good map is consistent application of a theme or concept; stuff like tengaku sticks really closely to its theme of ridiculous amounts of repetition in patterns, most hollow wings maps do similar things, and i strive to do the same in my own maps.
aside from the concept thing, the other really important part of a map is rhythm; if a map has bad rhythm, it doesn't really matter how good everything else is to me because this game is supposed to be about representing songs, not making stuff up randomly cuz it looks nice or plays comfortably.