What do I look for when I post here, constructive criticism on, the Mix and composition, nothing is gained by being destructive.  Which is what I try to do with most songs I listen to even if it just to echo what others have all ready commented on. 

Unfortunately I fall foul of the short comment, mainly because I can not think of anything constructive or meaningful to add.  When I do reply with a short comment like "Nice", or "Sweet", "thanks for sharing" whilst it may not help the person submitting the song/tune it does let them know someone has listened and that they found nothing to say.
 
Though I should mention the comment I do occasionally make, which from me means I listened all the way to the end, I liked the tune/song, I thought the mix worked really well and there is nothing I would change.  (If you do not get one it does not mean I did not listen to the end or that I disliked it)

There are some tunes I listen too which I do not like or something just does not gel for me personally, so making comment would probably be meaningless.  This does not stop making constructive comments though on mixes that are from a genre that does not appeal to me if I think I can add some value by commenting.
 
There is also only so much time in the day which means I miss stuff.

Here's my take:
We slave over these tunes, knowing they will not be heard by many.
We probably get our best responses from those who know the pain and sacrifice it takes to do this crazy passion of ours. The forum reaction is part of that calculus for many who post here.
I know I want to hear constructive criticism but I know my ego also wants to hear "awesome job, Jim."
This forum is populated with people like you who know how tough this can be and they appreciate your efforts. "Sounds great. Good song."
You will get that.
You also have people here with great ears, or are really exceptional players, or really excellent songwriters. They all come from those strengths with their comments. I am a fair to middling player of instruments so when some real pro player guys comment I read their reactions with that in mind.
To be real: you can be smart and take the mostly good with the occasional hurt feelings that some people's criticism can inflict (and I cop to doing my fair share of criticizing), or not.

I'd like criticism of the song itself rather than the recording of it.  I pay someone to do my demos,  I'm no engineer.  But it is nice to hear it because then I learn how to describe things to an engineer.  Also,  anything to do with sound is pretty interesting to me.