Build a community relational database or flat table that compares details and properties of data types specific to all RDBMS (dbf, sql, oracle, db2, xls.....) and ESRI formats (shp, mdb, axf...), even others like CAD/DXF and KML and rasters. This is too much detail to hold in ones head and is so important to data conversions and field mapping (especially in FME) and anything that consumes field values like label scripts. I have a flat table in excel that I pick at here and there but ironically, all these data type differences should be maintained in a relational database so that we can all know what to expect when going from shp to axf and back to sde, etc. It is maddening and I'm sure others have felt the frustration. It would be neat to see something like this included with the documentation that comes with ArcGIS.