[Stuart]

To me, the doctor should be be confident (almost to the point of arrogance), whimsical, sometimes moody, sometimes frightening, slightly excitable, but also vulnerable. They must also have a slight "alien" quality as if they don't always work with the same set of values we do.

Not all of the previous doctors had all of the qualities though, I'll admit. For instance, Tom Baker was confident (but sometimes arrogant), moody, excitable, frightening and definitely whimsical. Peter Davison was confident (but never arrogant), excitable, whimsical and sometimes vulnerable.

David Tennant actually played all those bits well, as did most of the other doctors (although not all doctors had all those characteristics). Matt Smith hasn't played all of them, but there is still time.