Yelp is one of the sites with some innovation and some critical thinking on their part. If you go there you may not see a large number of ads as expected from a site like this.
All they did was collected a bunch of serious critics, people who will be paid to write up about anything and everything online. Some people still try this themselves. They write huge reviews and hundreds and hundreds of them and then publish it all the time.
Yelp did just that, ego boosting for few who wrote reviews and a job for some. With this the build a huge repository of material and reviews just like a yellow pages stuff with reviews written on their side.
It was a good idea but made entirely to get public attention and traffic. It will be a good idea in India if somebody tries it, you will find one broadband users forum in India though which has a lot of traffic incoming. Also there are sites that are maintained only to give you reviews about movies and food and restaurants
Book reviews are not yet a hit in India.
There are a sites like Epinion that just survive on opinions. How original in their idea!!