Google Maps is now soliciting input from users as to whether reviews that are listed for a business are useful.
There has been an on-going discussion at convertoffline.com whether the recent purging of CitySearch reviews reflects a new effort by Google could to develop a way of indentifying more trust worthy reviews based on the number of reviews that a reviewer has provided. While that may or may not be the case Google appears to definitely be making an effort to determine review quality from user input by soliciting that information directly.
In the past Google has used review quantity not review quality as a critical ranking factor. Maybe that is changing.