I'm gonna go against the rest of the thread here with my opinion as someone who buys art, not sells it.
I think having some sort of built-in review system is good. Any website you go to that has a store front, a website where you can spend money, has some sort of review feature so that buyers can avoid people who are known to give subpar or bad service. Why is art any different? Someone said something like, "it will put artists at the mercy of reviewers" and...yeah, just like literally everyone else who sells goods and services to people. They aren't just reviewers, they're customers that paid you money, and deserve to have their complaints heard. I don't mean to come off as unsympathetic to the artist, but I personally feel that whenever these sorts of questions are brought up, these review systems opinions are almost always skewed in favour of protecting the artist above all costs and forgetting about the people who pay money for goods and services who can get screwed over just as easily.
As @Dunsparce above me said, while any review system can be manipulated unfairly, it's rare for this to actually happen if the system is implemented correctly with fail-safes involved. The ability to dispute a review that the artist believes to be in bad faith, for example. Another example would be a straightforward recommend or don't system (as in no bad reviews, only recommendations.) Like literally any other website on the internet with a rating system, people will learn to take it with a grain of salt and, most importantly, make their own judgement on who to trust and who not to trust.
Many people on TH don't ever touch the forums and have no interest to. I myself rarely check the forums. I would say a majority of users I talk to don't hang out on the forums, and mostly use TH to commission artists and keep their OCs in one place, myself included. I really think a review system would help protect these people from getting stuck in a crappy situation.
As for a rating being attached to a user's profile being seen at all times - that can be adjusted too. Maybe if a full commissions system gets implemented some day, the rating/reviews only show up if someone tries to apply for a commission using whatever system got put in. Perhaps if the two were to be implemented and tied together, you could further cut down on troll reviews by making it so that ONLY people who have actually used the artist's commission system on TH can leave a review. Sure, it cuts out people who are new to TH or commissioned off site, but it still covers a large swath of people.
As for public comments on reviews, I think the better idea is to force arbitration via PMs. Some kind of system where no one can reply to the comments or see the comments except for the first two - the person posting the review (who has the ability to change their rating at any time) and the artist being able to respond by saying something like "we have worked out the issue over DM!" Much like Google reviews or Etsy reviews. This cuts down drastically on public drama and forces people to talk to one another in private to work things out between themselves.
Sidenote: if everyone prefers to keep reviews to the PSA forums/threads, then I think a cohesive system might help cut back on some drama. See artists-beware on livejournal/their website; for years now this has been my go-to community for artist reviews. The moderation is strict, the rules are clear and concise, there's a template for all reviews (which some, but very few, people ignore), professionalism is a MUST (reviews with cursing and personal attacks are removed), and all around it's a good way to run an artist review site, I feel.
Just my two cents!
Yes I'm a year late to the party. D'oh.