I think there should be an option of a poll kind of thing where you can post a number in the area of concern. For instance. If you wanted to rate a sp, you could put a 10 for looks if you liked her, but if she was late without an excuse you could put a 6. That would make it possible to provide a review (of sorts) without getting so tasteless as I have seen elsewhere.
Another option could be being able to review certain aspects. I can see it being off limits to get into specifics about a womans body type (very subjective), but if she doesn't answer a phone ever, answers it during a session, or no shows on you without an explanation, I don't see the harm in reviewing that. I've only seen one provider (she was on time, attractive, very personal, and lots of fun) and it was a good experience. But when I tried to contact another, made the appointment a week in advance, called the day before and made sure we were still on, when it was the appointed time, no show. Disappointing.