Problem is, unless you take a balanced approach, credibility is lost.
How do I know the positive reviews weren't made by shills of the restaurant in question?
If you don't allow negative reviews, then I'm sorry but I won't be reading.
Yes, we all know it's a complex area, e.g. Restaurant owners lose defamation case
Surely, there must be some lawyers reading this forum who can advise how to build a review site which is lawsuit resistant?