Mixing a tune is all very well & good, but since most are here to learn & improve, I get the feeling that most people want feedback on their mixes... They wanna know if it's any good, great or completely hopeless, & if so why?, how can they improve?
What I'd suggest is that a condition of posting a mix should be that you give a critique of at least one of the previous mixes.
This is truly a win-win situation... Critiquing other's mixes is a great way to develop your "golden ears", & getting critical appraisal of your own mixes is one of the best ways to improve.
I can say this authoratively, because for the past 3 or 4 years I have regularly critiqued mixes on "Bash This Recording" over at recordingreview.com. In total I reckon I would have critiqued probably 2000+ mixes in that time, & it has been the best possible learning experience for me (outside of actually working in a professional recording studio, of course).
This is really a "pay it forward" situation - when it's done right, it breeds an atmosphere of respect & mutual helpfulness... It works fantastically over at Recordingreview.com, & one or two other sites have it happening as well.
I'd really strongly recommend this kind of initiative... it will save the site from becoming just another "look how much super-expensive gear I've got", & "look how great I am" sites that are unhelpful & just plain elitist. (...say no more!)