Since there's lots of Beatle talk at the moment, I'll start with a george harrison example: Run Of The Mill
Acoustic demo, perfect... very White Album-esque: http://www.youtube.com/watch?v=3ESGq841niY
Final version... production overkill: http://www.youtube.com/watch?v=BEA2pI41YVg
the great melody is drowned by needless horns, wanky Jools Holland-style piano, awful excessive drums... the whole production brings the song down.
Any other examples?