The author is not an attorney and nothing here is to be construed as legal advice
I recently sat in a Bar Association of San Francisco session on recent court cases regarding online reviews and slander lawsuits, spoken and moderated by an attorney from the EFF (Electronic Frontier Foundation) and the General Counsel from Yelp, as well as an experienced partner at a law firm who had extensive experience in such case precedence. All three people were very, very smart and obviously not only knew this space, but were actively a part of the process that was defining what is and is not legal, or what is and is not practical from a business point of view.
What struck me most about the case decisions that were developing in the industry were twofold:
1) A critical element seems to be about definition of who is the "Publisher." If a website is merely allowing people to leave reviews, the publisher is seen as the the actual writer of the review - not the website. This means that in such cases where the website is not actively promoting the negative reviews - they are immune from liability by a section of the Federal Communications Decency Act #230, or CDA 230.
In some cases plaintiffs (read: dentists) have been able to subpoena the website to give the information of the reviewer. However, this does not mean that the website really knows who this reviewer is. I mean, seriously - how easy is it to create a fake FaceBook account with a free-mail email address to reply to?
A public record of a dentist suing a patient for defamation and libel is about 25 to 100 times worse than a single bad review on the Web. If one thought a negative review in a social media website was damaging to their business, try having one of the local (or even national papers such as the Wall Street Journal) pick up your case as a mention in their ongoing fascination with online reviews.
2) There is something called an Anti-SLAPP statute that stands for Strategic Litigation Against Public Participation. What this means is that plaintiffs that sue reviewers in the hopes to intimidate them into removing those reviews - if found that the reviews are the honest opinions of the reviewer - could be responsible for the legal fees of the defendant. Talk about adding insult to injury. It's a move designed dissuade those with more money from suing those with less money into silence.