I'd say toughness / fearlessness. I like the fact that Rondo gets right up after he's knocked down, and isn't intimidated by other star players.
I'm a miniature Rondo in that

Hate being on the ground for longer than a second. I won't wait till I get picked up either.
Though, I have to admit, lately he's taken to lying around a bit, looking around, before getting up. Looks like he does it on purpose, to get the team to get back on D or something, to just buy a lil' time. He's had major minutes though. Dunno.
What I like about Rondo:
1) Yes, his dribbling is insane
The way he dribbles through traffic is just insane.
2) His quickness
Defenders *never* know what he'll do next. 180 Degree turns instantly, explosiveness in going up, etc.
3) His new-found consistency.
And this one comes with the consistency of his shot.
X) Yes, there's an X.
If there is *anything* like an X-factor for athletes like there is in performing artists, Rondo has it.