Hearthpwn

Hearthstone's entire history is littered with cards that someone, somewhere thought was unfair because it made a particular deck that they didn't like "overpowered".
And so the cycle continues...

Quote from TyrantrumRex >>

Quote from Scorpyon >>

As a card, yes it is fair.
Powerful? Definitely.
But pretty fair overall.

The loss of tempo is a big hit to take (and thats similar to what Keleseth does). 
However, what makes the card seem fair is druid's easy access to ramping.

 It's a big hit to tempo... Unless they also hit  Breath of Dreams on 2... Also considering the cheats... 

But, it's not a big issue with just Embiggen, it's a combo of cards. I will agree that it's a pretty loose combo, and when they draw right, it's quite hard to beat.

 Just to amend a typo - my last sentence should have been "unfair", rather than "fair".

Hitting Dreams, Embiggen AND the sidequest in the mulligan is a pretty tall order. And not very likely. 
If you get a God-draw in most decks, the odds of you winning increase dramatically, which is fairly standard and not inducive to Embiggen Druid specifically.

As a card, yes it is fair.
Powerful? Definitely.
But pretty fair overall.

The loss of tempo is a big hit to take (and thats similar to what Keleseth does). 
However, what makes the card seem fair is druid's easy access to ramping.