Seeing as Casanova was another RTD production I don't think timing was the issue.
RTD had worked with CE before, and worked with DT on Casanova. They're both actors he trusts.
CE is one of the finest actors of his generation and I was delighted he was cast first. He was a real "safe pair of hands" and I personally loved seeing him explore the lighter side of DWs character as that's not something you see CE do very often. I was really upset when I found out he was leaving.
Regular DW thread readers will already know I have a rather substantial crush on DT. I think he will have a very difficult time when he first takes over, for no greater reason than the new Doctor always feels weird for a bit!
He's also a great actor, with enormous theatrical experience and the ability to portray the gamut of emotion. So "in summation", I think they're both fab!
So what changes? I think the "formula" is reestablished in the minds of the audience so perhaps longer plot arcs (and hopefully more ambitious twists and clues - we'll be expecting it after Bad Wolf!). I think it would be fun to see a really prickly relationship between DT and BP - it would reinforce the great, almost romantic bond she has with CE is series. And finally, let him use his real accent!!!!