So I totally missed this until I was fighting Nightmare Mode Diablo.  And I feel dumb about it.  Since others will make this mistake since I did, you will want to turn elective mode on at some point while playing Diablo 3 so the game doesn't lock certain abilities in certain slots.  Elective mode lets you use any ability in any slot, broadening the possible ability combos you can equip.

Since this isn't painfully obvious (and it should be, since you need it), I hope you Diablo 3 players find this post.

Elective move is found under Options > Gameplay in the Diablo 3 menu.