Chris Lema makes the case for implementing WordCamp speaker reviews. I think it’s a great idea, and you should definitely read his full proposal. I do have some concerns on it though. For one, reviews are subjective, so organizers would really need to promote the review to keep them high quality. Also, some new speakers (of which there are a lot at WordCamps) may be discouraged by harsh reviews and be less willing to stay involved in a community. But I think these are challenges that can be overcome.
