Yelp is much more of a 'community' than TripAdvisor (or any other review site for that matter). A significant part of the responsibility for building and maintaining that community rests with their 'Elite' Yelpers, who are renowned for writing very thorough reviews, here's an idea of the length of some of their reviews (this one is one of 34 of a London business)...
A cursory glance at any Elite Squad reviews will yield similar results - they see a major part of their role as setting an example for depth and quality of review.
And they have lots of 'friends'. Some notes of caution for those businesses in the UK who have yet to be reviewed:
- Yelpers get a lot of kudos for being 'first to review'
- Once you have one Yelp review, you are highly likely to be reviewed by that reviewer's 'friends'
- Yelp reviews are shown prominently in Google search