But how can people's opinions written in the web be subject to any liability?
If you go around telling tell the world that in your opinion Fred down the road is a wife beater you best be sure you can prove it. Its not hard to find people who have gone to court and won damages for defamation. Doesn't matter if you do it online, down the pub or in the newspaper. Best think carefully about that hotel review you are about to post online. Can you prove the room was dirty? You might need to do so in court.