there's also a distinction to be made between suggestions that are based on personal preference (such as "how much reverb is right for this song?" or "how long should the soloist play?") and suggestions about things that are clearly unacceptable (instrument or vocals out of tune, bad timing, clipping, muddiness, volume mismatch between tracks etc)

I rarely comment negatively on anything I consider to be an artistic judgment call... but I have occasionally tried to gently point out the other types of problems when I hear them.

Never under any circumstance do I think it's OK to bludgeon, mock, ridicule or or otherwise defame a song or person who posted it. That's as bad practice from the audience as is playing out of key from the performer.