He didn't get into unity of setting, and unity of tone, but then, the genres were not clear enough perhaps to make it a distinct point.
Especially setting, since he didn't touch on world-building at all for some odd reason. But when trying to simplify my life introducing ideas to each other, perhaps the biggest condition is that they smell like they go together. Does this notion of time being shattered by a broken gemstone, so things shift about, really fit a plot where the founding principles of an organization, and arguments over them and the organization's continuity? Depends on the exact details of the plot -- but there are some notions that just don't work in a world where people move through time and can't control it. Does this ability to make the ground sprout with grass and flowers where you walk really fit with the harsh and nasty magic depicting elsewhere? It can be hard to convey because much of it turns on the exact details of the idea, and the connotations they carry. A story where wall panel, carved with wooden flowers, turns into a living hedge, can be either enchanting or horrific, but either idea would work with some, and not with others.
And you need variety in the setting. To keep the reader awake of course. Then there's the realism question: the world filled with all sorts of varieties of things, and a too uniform and homogenized world seems plastic.
You get that problem in tone, too. You need to write a comedy or a tragedy or a drama, and for that matter a light-hearted, frothy comedy, or a down-to-earth one, or what have you. But you also need to vary the tone -- with comic relief, or as Terry Pratchett put it, tragic relief. (Hmm. Can't remind whether he attributed it to someone else.) Hamlet at the graveside where they are making jokes about the staying power of a tanner's corpse. Even sharp contrasts. The scene in Mulan where the soldiers, marching along with a hearty marching song about a girl worth fighting for, march into a massacre site is superb. (Mulan is a very uneven flick, but that scene is one of its highlights.) You have to build, too, in the course of the narrative arc, and this often means adding gravity to the tone, and reducing levity. But still this all has to fit together. Hamlet, after all, was jesting about skulls and corpses, and wasn't going in for froth and silliness. Establishing early in a story that characters will die is important because it's breaking the tone of the story if you pop a death on us later.
one more thing to wrestle with. . . .