A good restaurant directory - some exist for restaurant niches, but none in general - could clean up with non-obtrusive ads if the focused on completeness and accuracy with a fairly minor feature set. It would just require having enough capital and time to establish social trust.

Curating reviews is just broken, just like SEO has turned out to be. It is unavoidably gameable. Yelp seems like a zombie site. They are free-riding on their prior reputation and gradually spending it away to acquire revenue. When the reputation is gone, the revenue will be, also.