One reason why I think people give bad reviews, is just to bump their post count. I've seen a lot of people who just run through BBC saying, "Good job, maybe *Insert one-ten word critique here*. 9/10." [I may be wrong here; this is merely my personal hypothesis.] If they only posted when they felt like they actually thought they had something useful to contribute, then BBC might be a better environment.
One last thought. I've been seeing an increasing amount of bad, and completely uncustom MOCs in BBC. I realise that a lot of the people are beginning MOCists, but there would be less if people offered constructive criticism. If someone make a one star MOC, but get praise for it, then they probably aren't going to try to make better MOCs. They're probably going to just keep making fifteen minute one star MOCs. Good constructive criticism is needed! Once again, good constructive criticism is telling someone what's bad about their MOCs, gently, and offering ideas on how to fix it.
Okay, I'm done for now. Now's the time to offer your thoughts on what I said. If you see any inconsistencies or errors, I'd love it if you would point it out.