There has been a Cambrian-like explosion of vertical search sites popping up over the past five years to fill the gaps that general purpose search engines like google and yahoo have not been able to fill. By specializing in a specific vertical e.g Yelp (for reviews), Kayak (for travel), Nextag (for shopping), these sites have been able to gather large audiences over the past five years, thanks in large part to the free distribution through natural search that google et all send to them.
It goes without saying that the result provided by yelp is better on many levels. This is simply a matter of focus. Google clearly knows that it can’t possibly server a result to satisfy every different type of user query, so this leaves room for other sites to specialize their efforts around a particular niche. Since not every search problem is created equally, these sites have been able to focus on building domain-specific functionality and filters to solve the unique search problems in their domains. This poses an increasing threat to the core businesses of google et all as these sites gain in popularity and start becoming the place where people start their searches.