Being a long time fan, I can say that getting used to a "new" doctor is always a bit of a tough thing. Especially if the last one was really good, which was the case with David Tennant. The first new doctor in the new series didn't do it for me, and I don't think he did it for the fans or the producers either—they say he brought a lot to the character, but you have to admit if they thought he brought so much why'd they get rid of him so soon?
The new guy will take a bit of getting used to. But it's happened before. It will happen again. The show has been on the air in one form or another since the 60s. There have been over 700 episodes, and there have been 10 other doctors.
David Tennant has also been one of the longest running Doctor's, so maybe we got a little spoiled...he was THAT good.