The old show was frequently highly reactionary but it also did better than most shows when it came to challenging establishment, bourgeois ideology and/or imperialist assumptions.
This division is the 'ethos'. Frequently reactionary but with a proportionately greater tendency to buck this trend.
The hero of the show is a white male with a professional title, a line in Edwardian clothing (which retains a formality despite veering between scruffy, dandified, bohemian, etc.) and who travels around in a symbol of the British state. The odd Jacobite aside, his companions are usually thoroughly respectable types.
So, even when he takes a moral line against exploitation, it can seem like the civilized Englishman taking it upon himself to explain ethics to the barbarians.
However, while it may be possible to characterise this as an "overall or originating ethos" (as a poster at Gallibase put it) it's one that has also been challenged from within.
At the start of the classic series, the Doctor is adamant that he cannot and must not intervene in history... including the religious practices of the Aztecs, a people destroyed by imperialism.
Then again, in that very same story, we also get a dose of condescension towards the Aztecs, portraying them as generally backward (i.e. "Autloc is the extraordinary man here!") and suggesting that their religious practices will shock Cortés into attacking them. 'The Crusade' attempts a very sincere portrayal of Arabs as human beings... but also includes orientalist stereotypes.
When the first Doctor intervenes in the future history of aliens, etc., he very often takes a stance that seems very anti-imperialist, i.e. in 'The Sensorites'. But, again, in that same story, the aliens are presented as encoded Asian stereotypes, and the human infiltrators are driven mad by their exposure to an inscrutable alien culture... which is pure colonialist self-pity.
But you also have to consider that, in the kind of fiction from which Who springs, the scientist figure, the lone inventor, was an ambiguous and untrustworthy figure who could not always be relied upon to toe the line. In Wells’ The Time Machine, the Time Traveller (clearly a forerunner of the Doctor) is explaining time travel to a group of friends when one of them imagines jumping forward in time to collect massive interest on a long-term investment… “…to arrive in a society run on strictly communistic lines perhaps?” suggests the Time Traveller.
All the same, the Doctor often assumes the right to intervene, which can seem imperialistic… but, having said that, the Doctor’s right to intervene does itself become the subject of some uncertainty within the show itself, several times. The Doctor has to justify himself to the Time Lords, firstly by claiming that power must be used to help those in need (and this in a story that forecloses on an imperialistic interpretation of that remark by being a forthright condemnation of imperialism), then by claiming during his second trial that he usually waits for a request for help from a local authority figure!
Times change and there was a shift in political discourse between the eras of the old and new shows.
Of course, political discourse was shifting - drastically - even during the run of the original series.
In ’63, before what we call “the sixties” got going, the show embraces the ethos of the post-war liberal consensus filtered through the tropes of the fiction which it draws upon. As the decade progresses, we get more attempts to engage with increasing social radicalism… getting more forthrightly radical as they go along, i.e. from the ambivalence of the anti-authority/pro(ish)-colonial ‘Macra Terror’ (there's a valid reading of this story that sees the Doctor as defending colonialism… though I’d point out that there’s no reason to assume that the Macra are the aboriginal inhabitants of the Colony… and that they also assume metaphorical valences that don’t really seem to include race) to the all-out assault on imperialism in ‘The War Games’… though, again, we see the divided ethos in the way even that story collapses into a weak reformism when the Doctor calls in the Time Lords and sends the humans back to their real wars.
The reactionary backlash is seen less than one might expect in the Pertwee era, possibly because of the left/liberal politics of Barry Letts… though he and Dicks inherit a framework in which the Doctor has become an adjunct to the military establishment. They cope with this by making the Doctor an infuriating maverick ecology-buff who scoffs at the Brigadier and assorted government types. Of course, the third Doctor is also very bourgeois in surface appearance. But he’s as likely to claim friendship with Mao (who, aside from his real odious historical character, was the emblem of a sizeable chunk of the European radical left at the time) as he is to claim friendship with Tubby Rowlands.
The show tends to trail behind the times a bit. There’s a time lag. So anti-Vietnam protests only faintly show up in the form of the Doctor’s peace sign in ‘Frontier in Space’. And the crescendo of strike action and union power of the early seventies only shows up in ‘The Sun Makers’ in ’77.
To just jump back a tad, I think it’s important to remember that the left was incomparably more influential in the mainstream during most of the original run than it is now. Thatcher and the rise of neoliberalism, together with the fall of what was called communism, dealt an enormous blow to left-wing politics in the late 80s and early 90s. The left is only really beginning to rally now. For most of the classic series, there was a rough ‘social democratic’ consensus in the country that progress was tied to social liberalization and a certain governmental role in investment and in curbing the power of business. Even the pre-Thatcher Tories accepted a form of this argument. However… and this is the key point… what we might call ‘social democracy’ was never really all the great on race and imperialism. A lot of Labourist thinkers assumed the inherent progressiveness of the spread of Western (white) civilisation. Liberalism was no better; often it was worse. Even Bertrand Russell was terrible on what used to be called ‘coloured people’ and colonization.
So, if the show evinced a divided progressive ethos (which I think it did) then that could be said to have stemmed from the divided, rising and declining social democratic consensus of the society that produced it. (As such, we’d expect it to be frequently reactionary, because social democracy was frequently reactionary on all sorts of issues from unions to race.)
The new show, of course, is a product of the wretched age of New Labour, of the rightward-shifted mainstream left behind by Thatcher, and neoliberalism triumphant… and yet, it produces episodes that are clearly ripostes to, say, ‘humanitarian interventions’… and even manages to correct its own lapses, with ‘Turn Left’ readable as a riposte to ‘The Unquiet Dead’ on the issue of asylum seekers, and ‘Planet of the Ood’ deliberately revisiting a moral lapse on the part of the Doctor regarding slavery and, in the process, becoming a parable about commodified workers that supports violent revolution!
So why the unusual degree of ‘bucking the trend’? Even up to recently, this was still happening (though less often and less reliably). So why?
I think its partly to do with the show’s roots. Take Wells, for example. He was a socialist, by his own definition. By the standards of his time he was a radical progressive. His templates for speculative fiction – The Time Machine and War of the Worlds – are, respectively, an allegory about class exploitation and a through-the-looking-glass parable about imperialism. And yet, he was (by our standards) a racist and a eugenicist (see what I was saying before about ‘social democracy’ being terrible on issues like race).
So, a divided ethos in embryo?
I think the subjective factor becomes important. Robert Holmes seems to have been an instinctive radical, at least in his writing - which is interesting given that his life shares some similarities with that of Orwell (i.e. Orwell was a policeman in Burma, Holmes was in Burma with the Army and then was in the police). RTD is also given to quite strong liberal/lefty critique in his writing... though he also seems influenced by the culturalism of, say, Dawkins and Hitchens and frequently flirts with a view of people that is pessimistic to the point of being reactionary. This is the left in the age of neoliberalism and the 'war on terror'.
These two figures in themselves - both apparently given to lacing their writing with liberal/left critiques but one working in the age of 60s counter-culture, a strong left, union power, etc.; the other working in the age of neoliberal triumph - may account for the different tone of the same 'divided ethos' in the classic and new series': the former leaning towards the left, the latter leaning towards the right.
Moffat, in my opinion, is a de facto reactionary by virtue (if we can use that word) of his sheer political disinterest and complacency, by his ironical raiding of political history for icons and motifs and nothing more. That could be why the show is now getting more and more reactionary, despite the fact that we are now moving - slowly and hesitatingly - into an exciting time of growing struggle.