I think Capaldi's Doctor won't be bouncing all over the place like Smith's was, but I also think he won't be overly serious or dark, he won't be a brooding Doctor--rather, he'll be a very confident and self-assured Doctor (even when he's majorly wrong about something).
I also don't think the Twelfth Doctor will be someone you take too
lightly, because he'll rip the baddies a new one if they push him too far.
I also expect him to be slightly
less physical than his most recent incarnations--rather than a lot of running around here and there, he'll have Clara do that. But I also wouldn't be surprised if the Twelfth Doctor has something akin to the Vulcan neck pinch (if not old-fashioned Venusian aikido) to incapacitate someone itching to fight him.
Other changes I'd expect:
- No bow tie (they are no longer cool with the Twelfth Doctor).
- A costume that's more contemporary than the Eleventh's current, but not as casual as the Ninth's.
- A new, but more modest sonic screwdriver (and less of a reliance on it). The Twelfth Doctor could have new tools in addition to the sonic.
- Brighter, more warm lighting for the TARDIS control room (but it otherwise stays the same as far as the overall design).
- New opening sequence and logo (or an extensive modification of the current one).