Just last week, a German friend of mine who lives in DUS (Düsseldorf) had tickets from DUS to ATL. He got to DUS, sat on the aircraft for four hours in DUS, then they got everyone off the plane. There was a mechanical problem that could not be fixed that day.

He was rebooked for the same flight the next day.

All he got for his trouble was a taxi voucher to get back to his apartment in DUS. No other compensation, no miles, nothing.

Edited to add: The airline was Delta code sharing with Air France.