Amir Khan's chin.

It has to do more with how you handle the punch that you dont see coming.
For example: Lucas and Danny were both getting hit with punches they didnt see coming and handle the punches well. If that would have been Khan, he would still be asleep. He wouldnt be able to take those bombs.
That punch that Lucas hit Danny with and the mouthpiece flew off, that punch would it kill Khan.
For the Maidana fight Roach knew that Khan was fighting a big puncher, they trained for it and Khan was aware of it. Plus it also helps Maidana being slow, Khan could see the punches coming.

 

Khan had Maidana tired from 9 hard rounds plus a brutal Body shot that is till don't know how Maidana survived and lasted the 12 rounds. Khan was also being trained by Ariza for this fight and was in tip top shape. ( I give Khan huge credit for this win, showed great heart and fitness)

In the fights that Ariza hasn't trained him in since, Khan looks really chinny again, even floored by Diaz Ha. With all that plus Maidana actually setting up his power shots with Jabs, maidana would probably stop him this time. What ever happens it would be another FOTHC, because of the styles involved.

 

It's definitely shaky. I think after the maidana fight he thought he could take bombs off anyone, despite the fact he was very very nearly stopped.

He was rocked several times against Diaz too who I don't think is known for his punching power.

That said, I'd like to see how his chin fares at 147 which he'll make easily.