I've had several shared data sources break after upgrading from 8.0 to 8.1 and wanted to shed some light and help others experiencing this. These issues below cause the shared data source to be unusable until the resolution is published.
Measures with a 'Date & Time' Data Type - For example, I had MAX([DateTimeMeasure]) as a calculated field in 8.0 that worked fine.
Resolution: In 8.1 a measure of 'Date & Time' data type is not allowed for some reason. You must change this measure from a 'Date & Time' to a 'Date' data type. (this was particularly annoying for me because I had to use this MAX Date logic to overcome an issue where Tableau won't accurately display the Data Update Time (please vote this up!): http://community.tableau.com/ideas/2761
Calculated Dimensions with IF/THEN or CASE logic using a Measure that is a Numeric data type. For example:
IF LEFT([MeasureName],2) = 'Ex'
Resolution: Change data type of [MeasureName] from a 'Number' data type to a 'String' data type.