I may be guilty of offering a lot of engineering advice, but seriously:
if the song doesnt do it for me, should I say nothing? Offer false praise?
If the singer croaks like a dying frog, they probably know it. They don't need me to tell them. They also don't need me telling them they sing like an angel.
Here is where I am coming from:
After a while, one pretty much knows the extent you can expect from regular posters, both in execution level and genre. And it is a pretty "giving" group. Even if it is not your cup of tea, don't they deserve something back? So instead of saying "great song, could listen all day" and be lying through your teeth, you can help them out with what you're hearing, and offer little ideas to make it better sonically. I much more want critical responses then praise.
As for the song itself: most times it is posted completed. You can offer ideas on arrangement (which having a DAW is not so hard to mute instruments or add an element), or on the mix. But the song is done...not many will take advice on moving choruses or changing keys or deleting intros or recording new melodic ideas because they have poured blood sweat and tears into it and THEY ARE NOT GOING TO CHANGE IT. I get so many good ideas for my tunes from the Forum and I still rarely have the inclination to re-do it.
Always be moving forward is my policy.