Yelp is the online giant for local business reviews. They have a rabid base of users that are quick to give their opinions on businesses.
Yelp along with Groupon can make or break your local brick and mortar business. Negative reviews on Yelp will show prominently on all of the internet search engines. Your potential customers are very likely to read reviews on Yelp, before deciding to purchase from you.
Also, coupon deal sites like Groupon make their decisions of who to feature and accept are based on your online ratings and reviews. So, negatives on Yelp can prevent you from being accepted into Groupon.
The controversy arises from business owners who feel that they are being extorted by Yelp. The business owners contend that the positive reviews are filtered and the negative reviews highlighted, unless they pay Yelp.
Yelp denies these allegations. However, their is a pattern of complaints from business owners that show that their may be some merit to the allegations.