Moffat thinks it’ll be fun seeing Clara cope with the Doctor being completely different…
…”I think the fun story will be – and we have the opportunity here – is this is what regeneration can do to you. He can be very, very different.”
Didn't Rose and a few other companions go through that already?
No, not really. Like I said, the differences among the past three Doctors have been minor compared to the differences between consecutive Doctors in the original series. They've all been relatively young, good-looking, and manic with a mix of brooding and comic attributes; the differences have been more or less shifts in emphasis. It's nothing like the shock Ben and Polly went through when the Doctor they knew as a crochety, proud old gentleman turned into a feckless, Chaplinesque eccentric, or what Sarah Jane and the Brigadier had to cope with when the elegant, mature, authoritative Doctor they knew turned into a madcap iconoclast spouting nonsense, or what Peri faced when the gentle, sensitive young Doctor she knew turned into a bombastic, aggressive, unstable egomaniac who tried to strangle her within minutes of his regeneration.