I am migrating a project between TFS 2010 and TFS 11. The problem is that the System.Description
field changed types from plain text to HTML between those versions of TFS and can't be migrated. To fix the issue, you simply have to exclude that field in your mappings:
<MappedField LeftName="System.Description"
RightName="" MapFromSide="Left"
valueMap="" />
The field seemed to contain mostly redundant information anyway, so it wasn't a big loss. See http://msdn.microsoft.com/en-us/magazine/jj130558.aspx for more information.