I agree there is a lack of information regarding good quality tutors and Merspi was originally going to go down the path of providing that service.
Feel free to take this idea and implement it.
An eBay for tutors. Registered tutors create a free account, post their ad, with no contact details allowed (or stripped), to force students to contact them via the site.
The site logs the hours they have billed (this provides an opportunity to take a cut of the money and monetise too***), and accepts reviews (maybe only after 3-5 lessons, except in the case of quitting beforehand, in which case it will be tagged like so on the review, for the other students to make their own judgements)
Reviews can be anonymous, and released at random times and dates to further protect privacy. (another possibility is only releasing reviews at particular times: at the start of each holiday period or something)
*** Infact, this is necessary to keep the integrity of the reviews, because otherwise the tutor could "tutor himself" via another account and then provide a ravingly awesome review. At least this way (s)he'd have to pay the system. Also, number of hours tutored with the student/tutor pair would be shown on the review (approximately, in a bracket, once again, to protect privacy maybe), but this value obviously increases the credibility of that review, and would probably be shown first or on top (hours could be even completely hidden, but then have reviews ordered by the backend).