Personally, and this is just me, I'd rather not have the empty positive comments on a thread. They're fluff and filler, and don't add anything. If I'm posting a MOC, I want feedback, critiques, and ideas, not smilies and generic comments. You can give positive reinforcement and still have a meaningful comment. Usually, it's just the difference of...
This is AWESOME!
This build is awesome. I really love how you used the engine cylinder to give a uniform shape to the side of the ship!
I didn't give critique or criticism, but I still gave feedback that's useful to a builder. My feeling of the MOC section is that it's to lay out a creation. To be fair, I don't comment on there as much I as I should, but also don't get to build as much as I'd like either. If someone makes the effort to post a MOC, there's an implied request for feedback.
That being said, the easiest way to get feedback is to encourage it yourself. Comment on other threads and builds, offer feedback to others, and that spurs them to repay the favor. As builders, we often sit in our own little bubbles, and don't provide the feedback we want in return. That's the easiest way to build up the section, and everyone can help.
You know, if I see links to Brickshelf, more often than not, the first thing I click is the "Back" button.