aristotle - 6:51 pm on Apr 15, 2011 (gmt 0)
According to what I've read, a major target of the Panda updates are large sites filled with numerous "low-quality" articles. These are sometimes called "content farms". In other words Google went after the "big fish", sites that show up in a lot of searches and are the main problem from their point of view. Most small sites are based on a different model and have a different purpose, so they didn't get caught in the net.