After upgrading to TFS 2012 users noticed that the Description field became read only in Excel. Before the migration they were able to upgrade this field in Excel without any problem.
Why is this field readonly?
I found out that during the migration to TFS 2012 the ‘System.Description’ field is changed from ‘PlainText’ to ‘HTML’. The advantage is that you now have rich text support when creating and editing the description field of your work item. The disadvantage however is that ‘HTML’ fields are not editable in Excel.