The thinking behind the changes, explained Stoppelman on the Yelp blog yesterday, is that "Lifting the veil on our review filter and doing away with 'Favorite Review' will make it even clearer that displayed reviews on Yelp are completely independent of advertising -- or any sort of manipulation," something the site has been accused of over, and over, and over.
I took a look at some of the filtered reviews on a few local places, and didn't see any blatantly obvious reason why they had been filtered out. Many seemed to come from users with few postings to the site, (which is sometimes an indicator of a shill or someone out to smear a place.) There were also a surprisingly large number of reviews being removed for violating the review guidelines or terms of service. Okay, not sure what this proves or doesn't prove about Yelp's business practices. If you want to peek behind the deep, dark filter curtain, go to the bottom of a business page and click the light-gray link for the filtered results next to the total number of reviews on the left.