I'm currently setting up an experimental 2013 environment with BCS, following the instructions in the AdventureWorks white paper found here - http://www.microsoft.com/en-gb/download/details.aspx?id=30398. Everything has gone well until adding an external data source connection in SP Designer (page 9 of the above).
Designer connects to the SQL database successfully (or at least, without error, and the database appears in Data Source Explorer), but the Tables / Views / Routines nodes have no content and can't be expanded. I originally thought this might be a compatibility issue because the AdventureWorks database is 2008 R2 and this environment has SQL Server 2012, but setting up and connecting to a brand new dummy 2012 database produced the same result.
Adddendum: The Operations Design View in the ribbon is grayed out and inaccessible.