If you watch the average episode of something like Star Trek TNG, they fit a lot of story in 45mins.
That's a good point. And more characters to juggle too. Interesting to think about what the differences are.
Something like the 3rd season's Ensigns of Command, where Data has to convince colonists to evacuate rather than try to fight the Sheliak, could easily be a Dr Who episode. But for some reason I'm mainly finding myself thinking of
terrible episodes that would fit the Who model (crew visits alien culture or human outpost, has to solve problem that arises, usually external): The Royale, Planet of the Joggers (aka Justice), Angel One, The Survivors, Devil's Due. A grim list!
Wracking my brain, there are definitely some good ones along these lines: First Contact (the episode, not the movie), Silicon Avatar, Darmok, Time's Arrow (a two-parter!)... I'm sure there are others, but what strikes me us how many episodes revolve around established races, violent conflicts and individual crew relationships and backstories, shipboard tech problems and the conflict of military matters and morality.
The actual current Who-type episode model of "introduce new setting - new characters - new problem - companion roles/development - Dr agency - solution and moral" seem surprisingly rare, particularly if you want the episode to be good. So many TNG episodes use familiar settings (the ship) and established-character-based stories (Federation admirals/corruption, Lwaxana, Q, Worf & the Klingons, Data's quest for humanity etc) that it makes me think that the one-off-play is a hard trick to pull off.