[Mark McLeod]

A late addition, here is a breakout of the 2010 teams that didn't return this season. The pie wedges are % of all of the 2011 season teams that dropped out.
The bulk (70%) of the dropouts had 4 or fewer years of experience.
The average experience of all 2010 teams happens to be 4.1 years.

P.S.
Here are the dropouts from 2010 and 2009 just so we can see that the trend in dropouts has been gradually shrinking for the 1-year teams over the past three seasons. 3 year teams also dropped a bit this year. Two seasons ago the bulk (70%) of the dropouts had 3 or fewer years of experience.
The 4-year team drop rate tripled this year alongside more modest jumps in the loss of 6 & 7-year teams.