Yup, it's the stats. He's a giant flyer who you can put into play out of nowhere potentially on turn four. This can really screw with your opponent's plans, because that early they may not be expecting such a formidable defense.
Moreover, Ao Shun won't actually die, so you're not wasting anything (though with the Surprise Defense nerf things are obviously worse as he'll get shuffled back into your library and you'll eventually have to find him again. The wealthy noble nerf also made it harder to play Ao Shun outright)
Considering how long it's been since I've actually played Ao Shun, does anyone know how he interacts with giant unstoppable creatures? (Karrck, Titan of the New World, some genesis monstrosity that was buffed and given unstoppable, etc) He's not dying, after all, merely exhausting.