If you operate in the US, there are absolutely no issues with Negative Comments.
Reason 1: US has very very loose laws in regards to libel, actually I honestly don't believe I have ever heard of a Libel case originating in the US. Majority is covered under the First Amendment, free speech / freedom of expression. In the US, a corporation is treated as a person, thus they are afforded the same rights under Constituation & Legislature (in most cases) as an individual.
Reason 2: Most companies in the US, which perform 3rd party services, are safe heavened. This means that if you are a review site, you act as an intermediary, a 3rd party. You connect a seeker with a commenter, not post the actual comment yourself. The good thing is, most of the time, the worst that can happen to a safe haven company is a court order to take down the material. Cases like that will be rare, basically only in a case where it can be provn the information is blatantly false and was posted for the sole purpose of defamation.
That said, other countries have different framework. If you are a British citizen, you may be responsible for actions of your US company, even though it does no business in the UK. Although, I find this extremely unlikely, and I'm sure there is strong framework in place to prevent such a thing.
All in all I think you should be totally fine. I provided some links for you to read up on, they are Wiki pages, but on the bottom you will find links to the "legalese" versions on the Government Printing Office site.