OK, now we know that the superb Matt Smith is to be the 11th Doctor, here are my thoughts on how he should play the role:
1) Bring back the flamboyance of Jon Pertwee. Play the Doctor as a charismatic young dandy.
2) Incorporate elements of Peter Davison's portrayal, especially the sense of justice.
3) The costume: something with a hint of Oscar Wilde about it: flamboyant and a tad 19th century.
4) Emphasise the Doctor's darker side and sense of loss, but not all the time.