You are missing the fact that a double-faced card that enters the battlefield enters with its front face up by default (C.R. 711.8). (The last ability of Nicol Bolas, the Ravager overrides this default, since that ability specifies to "return [Nicol Bolas] to the battlefield transformed" [C.R. 711.8].)
Thus, if Nicol Bolas entered the battlefield from your graveyard, he does so with his front face up (that is, as Nicol Bolas, the Ravager) by default, since nothing in this scenario states otherwise. This is so no matter what face was up when a previous version of Nicol Bolas left the battlefield. However, since Nicol Bolas, the Ravager is a creature, his enters-the-battlefield ability won't trigger as a result of Hushbringer (C.R. 101.2).
It sounds like a bug if the situation is as you described. Hushbringer only interacts with triggered abilities and Nicol Bolas's transform ability has no triggers at all.
Opponent has a Hushbringer in play. I have a Nicol Bolas, the Ravager in play. I pay 7 mana to transform him. He ends up in the GY.
I assumed he would come back as a Planeswalker since the wording says "return him to the battlefield transformed".
What am I missing?
Yep, sounds like a bug, unless there's some other card messing with things which you aren't aware of.
@Peteroupc; you seem to have misunderstood the OP's question.
Opponent has a Hushbringer in play. I have a Nicol Bolas, the Ravager in play. I pay 7 mana to transform him. He ends up in the GY.
I assumed he would come back as a Planeswalker since the wording says "return him to the battlefield transformed".
What am I missing?
Thanks for the help!
My 450ish 1vs1 cube
You are missing the fact that a double-faced card that enters the battlefield enters with its front face up by default (C.R. 711.8). (The last ability of Nicol Bolas, the Ravager overrides this default, since that ability specifies to "return [Nicol Bolas] to the battlefield transformed" [C.R. 711.8].)
Thus, if Nicol Bolas entered the battlefield from your graveyard, he does so with his front face up (that is, as Nicol Bolas, the Ravager) by default, since nothing in this scenario states otherwise. This is so no matter what face was up when a previous version of Nicol Bolas left the battlefield. However, since Nicol Bolas, the Ravager is a creature, his enters-the-battlefield ability won't trigger as a result of Hushbringer (C.R. 101.2).
EDIT (Sep. 22): Struck out.
@Peteroupc; you seem to have misunderstood the OP's question.
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules