I've wondered about this a bit. I know it takes time to critique others, and some may feel they don't have enough knowledge to offer criticism, but I think we all listen to enough music to know what sounds good and what doesn't. A critique doesn't have to involve technical analysis, or suggestions for how to improve, just some notes like a band would give can be helpful, along the lines of 'The bass seems boomy" or "I can't feel the kick."
If anyone expects to get some critiques, they need to be giving them, too. That's what makes a site like this useful, in my opinion!