Already mentioned on the main page, but the fact that Jamie is able to resist cyber-conversion in Academy of the Cybermen thanks to the sheer loyalty he has to the Doctor and the strength of his emotions.
Not the first time he's managed that either (from the order in which they were written, at least). In The Best of Intentions, he breaks the Great Intelligence's control over him even more easily. As pointed out in the story, other characters the Doctor had met in-universe weren't able to do the same.
