If someone doesn't specifically ask for feedback, I'm not going to give any. Here are the typical ways that can go:
A: Here is my song
B: I don't like X about it
A: Well, I didn't ask what anyone thought
- or -
A; Here is my song
B: Love it!
A: Thanks!
What's not represented is:
A; Here is my song
B: Love it!
A: Well, I didn't ask what anyone thought
So by this law of nature, if someone posts a song and does not specifically ask for feedback, the only "polite" response is "love it!". Although this might not be the honest response. If I'm really honest, I don't "love" much music I hear at all, here or anywhere else. I "like" a lot of it, but the enthusiasm hoped for from a poster is rarely my most honest response. And I'm certainly not going to reply to a post with something such as "I like, but don't love, this song". Maybe what I should start doing is saying "I listened to your song. Thank you for posting it." :-)
If I ask myself honestly, the more I like a song, the more I have to say about it, because I'm --engaged--. I hang out in forums like this because I find it endlessly exciting to hear music that is maybe unfinished or a step on a longer journey. These aren't products, they are living art. If I want to hear the greatest music in the world, I have my iPod.