Defamation is a very serious criminal offence and it is not as difficult to sue someone for damages caused by it as you might have been led to believe.
For a start, you cannot rely on your qualifications as a consumer to criticise a business with your opinion. To be a believable critic you need to prove that you are qualified to make the criticism, you cannot express your opinion based on a single personal experience even if that single experience is true it does not reflect over the entire business. So how much worse can it be when the statements are false?
Search Google for Tripadvisor legal action and see for yourself whats happening everywhere. Even in the USA they have changed the law to ease the course of action for defamation. Check the following websites they may be of help to you if you have a case for defamation against Tripadvisor or even others:
By the same token, comments praising a particular business may also be false; they could have been placed there by the business itself, friends or others in exchange for a free meal or accommodation. The sad part about this is that Tripadvisor has had too much money to create a web market that is denying appearance on searches to other bona fide companies that deserve it.
All I can say is that restaurants and hotels should discourage this website and request their entries to be removed. And yes,
individual persons cannot hide behind the website with the pretext that they have been encouraged to express their experience.
Their experience can actually cost unjustified serious losses to the company concerned which is certainly able to sue for damages, especially if it can be proven that the information was deliberately malicious.