Strong female characters
One does not even need lead characters to be female to have a significant presentation of strong female characters. I (and I suspect more than a few male nerdy science fiction fans) enjoyed the early, highly intelligent presentation of Romanadvoratrelundar (sadly the character later became "just a companion"). Having a peer (superior?) in intelligence albeit with less experience provided nice opportunities to show the Doctor's strengths as well as present a positive female role model. (What other kind of character would be able to say both as a jibe and respectfully that the Doctor wins by making mistakes?)
While Doctor Who is not exactly known for consistency, it would have been nice if the established expectation that regeneration does not change gender (which was changed at least as early as "The Doctor's Wife"). It would not have been difficult to make up some technobabble to explain the exceptional case, but those managing the series chose to support the an arbitrary view of gender (which is distinct from equality in worth).
(The series also seems to have diminished the superiority of the Doctor. Part of this is normal (bad) enemy/conflict inflation (saving worlds, galaxies, and even universes can become old hat), but I suspect part of this comes from trying to make the companions more significant. This could have been done in other ways than making human companions peers (or superiors), but it is easier present quantitative value than qualitative value.)