Comments on open mic....
Key here is respect, I think. It is earned but at the same time should be given. I'm going to assume that the songs there are "finished" and whether I agree or not, I need to respect the artist and treat it as such. Perhaps they're simply sharing the songs and are not even wanting or needing critique. Comments should be encouraging (I feel I'm preaching to the choir...lol) unless one has that relationship (there's that word) that allows for critical honesty, and, like Phil mentioned, kept to a private conversation.
I haven't said anything everyone doesn't already know, but I think it bears repeating now and again. I think no matter what, or who, we all need a filter installed between our brain and mouth, in spite of our best intentions.