That would be a great thing, Larry! A key extra part of the question is "compared to what original data source?"
Pretty much all my Tableau experience is with JET sources, here's what I know for JET data sources (Excel, Access, text files):
- extract gains COUNTD, MEDIAN, PERCENTILE (new in 8.1), using IN/OUT of Sets (new in 8.0)
- ROUND() can return different results, particularly for STR(ROUND()) when applied to aggregates
- SIGN() can return different results
- extract loses RAWSQL functions (true for all data sources)
Also, I've found differences when moving between a JET source, an extract, and an extract published as a Tableau Server published data source, I'm not aware of any that exist today but my experiences made me limit my use of published data sources.