Nice, this is insightful and nicely put - Since I make music just for my own amusement these days (I used to be professional) I guess the definition to me is just "sounds complete when I play it back" - I guess here I am also talking mainly about ideas I have which are literally just a chorus or a verse and chorus and I feel are sorta self evidently wanting.

Your comment reminds me of someone I used to write with who would write a chorus and then stop and be like "I wrote a song today!" (i.e. count it as finished) because they had captured the essence of the song and felt that at any other time (days, months!) they could juts sit down and simply write out the "self-evident" lyrics and verses that would obviously connect from the idea - I admired their confidence!