I agree with the posters who said that negative reviews are far more valuable than positive ones, at least for me. As a relatively new punter, I will avoid wg's with negative reviews (unless it's one of those that have 50 positives and jus one or two negs....), and I generally will only see highly rated wg's at this point, which is pretty easy to do in London.
I'm guessing this is why most of my reviews have been positive so far, as I'm not really much of a risk taker. I don't feel like I'm good enough at figuring out who to avoid and I'm probably not ready to walk yet when perhaps more experienced punters would at the beginning of a punt. I would have no hesitation at giving a negative it is was warranted, however, and my first review here was a strong neg, but that was before I discovered UKP, so I learned my lesson the hard way, lol...