Interesting thread. I find that I 'listen' to songs in batches, like this morning, and I've commented on most that I heard, but I did not listen to all of them all the way through. If I like the style or genre, I usually do listen to the whole song. If I comment, I try to say something positive about the song, or the mix, or the musicianship. On occasion I offer a suggestion, but as a relative newcomber and novice, I'm not really comfortable doing that or consider myself knowledgeable enough to.
I think I can tell from the comments on my songs that some are essentially encouragement, 'Good so far', 'Keep going', while others are much more than that. It never occurred to me that a poster didn't listen to 'any' of the song though.