Let's try to keep things friendly around here, everyone's opinion is welcome, but no need to get personal.
I don't know a good way to really encourage people to post feedback. I mean, for me, posting feedback is a great way to learn, as it forces me to think a little more deeply about a mix and why it isn't working. I listen to mixes on the radio the same way, and often hear things I would have done differently.
At the same time, I don't have time to listen and critique a lot of stuff, just as I don't have time to participate in as many mixoffs as I'd like.
I can write a little 'Guide to Giving Feedback' Sticky for the 'Welcome' area, and link to it from the 'News,' maybe that will have some effect.
Giving feedback takes time and thought, and many people (especially newbies) are probably hesitant to criticize when their own mixes aren't great yet. And you never know how someone will take being told their mix is pretty weak. Most seem to take it well here, which I'm proud of, but you never know.
Even those of us who get paid to mix are always learning, and trying to improve.