For me, quality depends on how polished something looks. It's all about execution and not necessarily details, style, or even cleanliness in regards to certain areas...
Cartoon or simplistic styles can be fantastic and have amazing stylised anatomy. I honestly love cartoon styles as the expressions and movement can break rules of reality. Detail isn't a deciding factor and can make certain styles muddy. I prefer something created with flow and purpose rather than a lot of unnecessary parts.
Messy paint strokes can be a wonderful addition and lines that aren't super crisp can be good sometimes. A good basis of lighting does help a lot, as does colour theory. Lighting doesn't have to be crazy, and sometimes flat colour work too. Lines don't have to be thin or crisp - bold lines can work for certain styles. Sketches can be high quality. I consider the upper left part of the following piece to be the highest quality I've ever commissioned.
I will say even the quicker sketches are very good quality because the anatomy works, the lines are purposeful, and the flow/balance is understood. And I must say, this isn't even her best work. She managed an older character fantastically despite drawing young adults most often. Slight tangent, but I wanted to share...
Alternatively, here's a style that is neither semi-realistic nor anime that I adore and consider HQ as well.
btw I recommend both of these artists with my full enthusiasm. Wonderful people.
Basically, any style can be HQ! I think it's about time, execution, effort, and practice. I don't think someone has to work as a professional artist to have HQ skills.
I put myself in the medium-quality range. There are people out there who have much stronger line work, flow, and have lighting on point. And man...when an artist can do hands well - I'm in awe. I definitely study pieces with qualities that I'd like to improve upon. That being said, I can see high quality being disheartening, especially for artists who are still developing and would like to sell. I can also see it being tough on artists who don't do anime or realistic styles since those are the most praised styles.