(Assuming people haven’t been asking this loads on here already!) Considering the programme is known for its ability to change settings, places, times and peoples, genres, sometimes quite a lot in one episode. If it jumps around so much, what can be said to be something that makes the programme, unique to others, its own mark? If it is those things which are most commonly shared between stories, is it simply the TARDIS, the Doctor and companions? Neither the TARDIS nor its inhabitants appear in every episode - Mission to the Unknown has none, granted that it a one-off, and yet a precursor to a longer story, and still credits Hartnell, but then how far can the programme change and retain the name? This accepted level of variety seems unique for each person. Or, is it the very fact that it does change so often what makes it unique? Is the fact that the programme changes settings/characters/morals so often relevant to the quality of those individual stories? Longevity was the leading principle in its creation, before even the genre was decided on. One must recall how swiftly the central ideas of historical education and the rule about there being no bug-eyed monsters were changed, meaning even if one takes the very first series as a guidance on how the programme ought to be, it is made under a constant sense of not enforcing its own rules.
Comparisons have been made to other often-retold stories of folklore, Robin Hood, King Arthur, et al. Unlike those stories, however, the programme on television is made to be one big adventure, as witnessed from the beginning back in the sixties, all the way to the present day, so it isn’t so much literally re-telling its own story as telling one ongoing story, leading to an inevitable degree of change - but must it always revert back to the Doctor travelling through time in the TARDIS? Seems simple, but maybe that’s all there really is.
As soon as you get into any kind of specificity beyond that, anything besides the apparent fundamentals, it opens up further and further room for discussion, disagreement and debate - maybe there’s no notable difference between asking what makes a good Doctor Who episode and what makes up Doctor Who itself, both equally subjective.
Whether Doctor Who is “political” is a similar kind of question - some claim it always has been, others say it hasn’t, it shouldn’t purport to be when escapism and adventure are what it provides best. Whatever any author writes can be seen how ever any audience desires, though. One person’s idea of what makes a good episode of Doctor Who may be fundamentally at odds with another person’s view. Anyway, this is just something I think about (mainly about the TV episodes), I wonder what other people would say.