Reviews on sites like Amazon or Yelp are a monoculture: everyone sees the same set of reviews and the same ratings. This creates leverage that makes it worthwhile for shills to spend large amounts of effort "infecting" those sites with bad information, since once they find effective ways of doing so it affects everyone.

So the solution must be a review system that works like Twitter, where each user has a unique "view" composed of sources they've selected, directly or indirectly. This diversity would make infection much more difficult and less rewarding for shills and other attackers.

I'm still trying to figure out the best design for such a system. The requirements and usage patterns would be quite different from Twitter's, and I'm not aware of any existing attempts that I can learn from.