Tripadvisor is turning out to be a key determinant factor in the success and failure of hotels and restaurants. In fact, a recent article on the BBC was commenting about how bad reviews can really kills entire businesses.
http://www.bbc.co.uk/news/uk-16501967
And interesting challenge would be to understand how to prevent unfair ratings from untrustworthy tripadvisor users, and it would be also relevant for many other open crowdsourcing systems where there is no control on the type of user that posts information. It is actually possible to access the tripadvisor data via an api.
http://www.tripadvisor.com/help/what_content_is_available_for_free