Like the people above me have said, it really just depends on who is evaluating. Everybody has their different likes and dislikes. One person may absolutely hate something you make, while another may fall in love with it. Though, I think that the best way to evaluate something is to put your own preferences aside and do some empathizing. This means you put yourself in the producer's shoes. Try to look at what the artist was going for. Do you see the same motivation they had? Sometimes, it is also best to put more though into it than the original artist did. Can you imagine why the artist would take the time to make he or she made? What is he or she trying to prove, if anything? Then after you ask yourself all of these questions, then perhaps you may jump back to your likes and dislikes. Of all the thought you've put into the evaluation, does the piece before you please you? Why or why not? What could the artist have done differently?
I guess this is just an example of one of my ways to evaluate stuff. I know, it kinda sounds like a bunch of Bull-Huh, but I think it is effective.
Unfortunately, people on this site tend to evaluate art based on the if the art has giant breasts or bubbly butts in it. Not to be a hypocrite or anything, cuz I draw a lot of that stuff, but, damn, some people are just so anal about porn. I mean, it isn't even art to begin with. But I digress.
So what I'm saying is, evaluation should have a bunch of critical thinking done in it. It should point out the negative as well as the positive. In the evaluation, one should not try to change the artist's style, no, rather, they should give tips on how to improve. That is the key. There is no such thing as the perfect artist. But, by golly, you can sure as hell try to get pretty damn close to perfection.
I hope this helps~