This is, of course, the first episode to be credited to Dennis Spooner, who will go on to be a major creative force on the series. It's safe to say that the difference is immediately noticeable--the previous historicals, with the exception of Coburn's opening script (which is not so much a historical as a prehistorical anyway) were all by the same gentleman, John Lucarotti, who took a view that the essence of the historical was, well...history. 'Marco Polo' and 'The Aztecs' felt very much like stories that could not have been told at any other time or place; 'Marco Polo' centers on Marco Polo and his efforts to return to Venice to the point where he gets the concluding swordfight against the villain, while 'The Aztecs' keyed in on the clash of values between the 20th century European viewpoint espoused by Barbara and that of the Aztec civilization. Even when Lucarotti wasn't being out-and-out educational, he still focused his writing on the importance of the setting to events.
Whereas here, Spooner is already making it clear that he's less interested in history as a series of events as an aesthetic. The Reign of Terror is important to 'The Reign of Terror', but it's important as a setting and a collection of tropes. It's a backdrop for the latest exciting adventures of the Doctor and his companions, a collection of snarling peasants and sniveling aristocrats and mentions of Madame Guillotine. The fiction of the French Revolution is almost as important as the fact, which explains why it's episode one and we're already getting Rouvray as a sort of pseudo-Pimpernel. (Only a much less effective one. "They seek him here, they shoot him there, he lies on the floor and bleeds everywhere!")
The change is evident in the efficiency of the scripts--as an evocation of a time, a place and a specific culture, "A Land of Fear" is nowhere near as solid as "The Roof of the World" and "The Temple of Evil"...but as a mechanism for getting the Doctor, Susan, Ian and Barbara separated and into a series of entertaining predicaments, it's lightning-quick and brutally effective. The pace of this episode even by modern standards, and it's no surprise that many of its tropes were still used decades later. (Set "A Land of Fear" next to Part One of 'The Visitation', and it's like looking at twins.)
It's more than just the pacing, it's the attitude as well. 'Marco Polo' and 'The Aztecs' got the main cast into period garb as well, but there's nothing quite as charming as the way the cast upends the trunk full of costumes and gleefully plays dress-up. The previous stories also had their hints that Ian and Barbara might be on their way home, but never played with such rueful comedy as here. (And it's worth noting that Spooner's first script is also the first point where Ian and Barbara admit, if only to each other, that they're not exactly disappointed at the thought of continuing their journey on the TARDIS.)
If you did want to make a bit of a complaint, it's hard to ignore that this approach is a bit of "does what it says on the tin"; the downside of valuing the setting more as a backdrop for the Doctor's adventures is that, well, it reduces the setting to a backdrop for the Doctor's adventures. It's no wonder the pure historical starts becoming less important from here on out--if the Doctor is just doing "Doctory" things in a variety of settings, then surely it's hard to argue that it's not more fun to include monsters and aliens in among the famous historical figures, right?
It's also the point at which you really start to understand just why Carole Anne Ford wanted to ditch the show. There are a lot of scenes in "A Land of Fear" where she just stands there staring at the camera while everyone else is talking. She makes the most of her lines, and certainly gives her all when the script gives her something to do, but it's really noticeable here that the writers just aren't as interested in her as they are in the other characters. She's still got fourteen episodes to go, but the writing is on the wall--all Susan seems to be good for anymore is to shout "Grandfather!" at the episode break. And the production team is starting to realize that pretty much anyone can fill that part.
Well, metaphorically at least.