I was told when I first started graduate school that it's not about how smart one is, but rather it's about perseverance, the ability to jump through bureaucratic hoops, bite your tongue and do what you're told.
I have come to believe this since I've met some incredibly brilliant PhD's, but I've also met some incredibly dumb PhD's.
A PhD is more about elbow-grease than intellect. You do need some intellectual chops (in your field - you can be very naive outside it), but you need perseverance more than anything.
I've met some PhDs who would run rings around anyone in their niche, but would be (metaphorically) unable to tie their shoelaces by themselves.
I have come to believe this since I've met some incredibly brilliant PhD's, but I've also met some incredibly dumb PhD's.