This Daft Old Face
A couple of final links for old time's sake. Firstly:
Semi-satirical investigation into the Doctor's political leanings using statistical analysis, assuming that real world events are mirrored in the Whoniverse (the author clearly hasn't been within a dragon's breath of AHistory, either volume). Lawrence Miles addresses this very point at some length during the Interference epic in which the character is called upon to explain exactly why he's quite happy to fly about time and space knocking off abusive governments but when it comes to other regimes, particularly on Earth he's rather more circumspect.
I don't remember an answer really being forthcoming other than some deep dish anguish although my understanding has always been that if the Doctor is aware of a status quo before he gets there he can't change it without there being consequences (see Father's Day etc.) but if the situation's totally new to him he can rebel away. Which would explain why he's never specifically gone to meet a young Hitler and tried to persuade him to persevere with his painting (ala John Cusack in the film Max) or whatever. Plus it seems to have been established that Earth has a special place in the web of time, on one of the causal fault lines.
Which is very clever even if, as Steven Moffat would point out, the windows are the wrong size. If this was dimensionally transcendental it would be a neat way to store your entire wardrobe without having to build an extension on your house. After all, doesn't Tardis actually sound like the kind of product name IKEA would give their furniture lines? In fact most of the current real furniture names -- Pax, Hemnes, Aspelund and Mongstad -- all sound like characters from the new series.
Which at first glance would seem to have nothing to do with Doctor Who (other than to say she'd make a smashing companion), except the paper that she co-wrote, "Frontal Lobe Activation during Object Permanence: Data from Near-Infrared Spectroscopy" is about looking into child's brains to find out how they work. Imagine the Fear Factor and the fear factor if in future the BBC simply recorded infant brain activity to discover how to scare the bejesus out of us...