The new Doctor Who serials are fantastic. They take a look at Doctor Who in a new, slightly emo light, and show us the pain of a guy who may have been the end of his own people.
This is a traveler in the universe who is ultimately alone, and to whom no true companionship can ever be held. Perhaps this is the reason for the major change in the role of the Doctor's "companions."
In the old serials, the companions were essentially children. Even when they seemed relatively mature, the Doctor still treated them essentially like the child assistants on Mr. Wizard.
They were there to learn, and to observe, and almost always to be talked down to Watson-style (Mary Jane is the notable exception to the passivity of the old series companions).
More frequently than not, the plot revolved around the companion getting into trouble, and needing to be rescued by the Doctor, and while the companions did seem to be overwhelmingly female, there never seemed to be any form of romantic interest between the characters. That's simply not what they were there for.
Starting with Rose Tyler in the First of the new series, we see something new with the companions.
For the first time, the Doctor seems to notice just how pretty one of them is, and for her part, she gets some real independent character development, and often enough it was the Doctor who got into trouble and needed rescuing.
In fact, the over arcing plot line, even sees the Doctor in need of rescuing on another level, which Rose is ideal for, seemingly pulling him from his introspective turmoil caused by the destruction of his planet and people at the hands (laser pincers?) of the Daleks.
The Doctor and Rose developed a strong emotional relationship, and after she left the show, the trend continued.
Martha was less obvious about her desires for the doctor, and Donna's Doctor/Donna' storyline was much more compelling than the hinted at romantic possibilities, but Amy's interest in the Doctor was the most blatant and focusing. The entire time with Amy seemed to be working through the romantic possibilities, while the audience was rooting for her to come to her senses and return to her fiancé.
We'll have to see what happens with the next companion (who knows, maybe it'll be a robot dog), but we already know that we'll see more of River, who has an obvious romantic past/future with the doctor, so the trend is not one that will be soon abandoned.
Is this a good change for the canon of the story? It's hard to say. It certainly makes the Doctor seem more human (yeah, I know he's not human) and vulnerable, giving his character a dimension it never really had in the old series, but on the other side of that coin, it trivializes the companions themselves to an extent.
We know that they are all temporary additions to the Doctor's life, and must eventually part ways, making any real relationship impossible, and if the writers aren't careful, they could turn the doctor into a Captain Kirkesque kissing machine with no real substance to his emotion; a man whose every romantic encounter is a joke or device.
[Feature image owner: BBC, in-text image owner: unknown/public domain]