Whether by accident or design, each new series has had a new headline-grabbing element at it's start - a new Doctor or companion. This always helps to keep the show fresh I suppose.
When Christopher Ecclestone quit, RTD and the gang wanted to keep it secret and spring a surprise regeneration at the end of series 1. That didn't happen, of course. Now I can't help feeling they'll try harder next time - so it almost feels inevitable that DT's departure will be a 'surprise' one (depending on leaks).
And so, the Christmas special and start of series 4 will once again have that headline-grabbing element to it.
Personally, I'd really like to see DT do a traditional three year stint.