I love the idea of having an eye-catching clue for an image where serious critique is requested. 

Perhaps even a badge or colored strip on the thumbnail that's then repeated in the comment box?  Something obvious that stands out would be good.  I NEVER look to see if people want constructive or only happy comments unless I'm feeling moved to give a crit.  When someone asks for crit, I do what I can. 

From time to time I specifically request feedback and contstructive crit, and get a bit frustrated by the lack of response.  I'll admit, most of the time I'm just posting things I like, stuff I consider pretty or interesting or whatever.  But when I'm trying to learn something new, I'd love to hear things like "you could reduce the grain by setting such and such a setting somewhere between this and this."  I don't know much about photography from the technical side, and the camera I use has its limitations, so I just sort of learn as I go.  Reading a technical article about how the camera works will go right over my head.  Learning some specific little thing is what moves me forward.

Like Ronnie teaching me about low light photography.  I didn't know how to do that.  He told me one or two things, and now I'm sure you all wish I'd just stop!  LOL!  But I DID learn something that allowed me to advance my skill level, even given the constraints of my camera.

So if you guys see me asking for constructive feedback ... please! ... I'm serious! 

"If your pictures aren't good enough, you're not close enough."  ...  Robert Capa