In the Motown catalog, recently I was thinking (actually not just recently but I listened to it recently) that What You Don't Know could have been a powerful song about various more serious topics... but instead they made it one more random love song...
So I don't know about single releases and promotions... not things that I usually care much about... but it would have deserved better lyrics !
(If I'm not mistaking it was also one of Michael's favorite on Dancing Machine or on that-solo-album-that-nevrer-got-released-and-became-Dancing-Machine-and-Farewell-My-Summer-Love <- facts to be checked.)
In more recent career, songs that I think would have deserved more attention (like being completed back then and/or released) (non-exhaustive) : People Of The World , Seeing Voices , Dream Away , We've Had Enough , Scared of the Moon , Gloucestershire , ...