Growing the Beard: The first book has a rather more childish, Enid Blyton-esque character to it, with the major themes and tone of the series established in book 2.
Will and Bran. Possibly Will and Merriman too, given that Cooper originally intended King of Shadows to be about a love affair between William Shakespeare and a boy actor. Note in Silver on the Tree, where Bran comments on the prettiness of Jane Drew, which Will as an Old One seems completely oblivious to. This could be seen, however, as the author throwing in a Have I Mentioned I Am Heterosexual Today? moment to discourage such thinking. It should also be noted that by the time of the last books, Will has very much settled into his role as Will the Watchman of the Light, with his humanity openly acknowledged as more protective coloration by this point. Poor Bran seemed very delighted to meet mortal kids in the last book. Being an Old One's best friend=not easy.
King Gwyddno and Gwion from Silver on the Tree have this in spades. The insistent use of "love" and its derived terms when describing their interactions in the chapter in which they reunite makes it seem quite deliberate.
The Mari Llwyd in the final book, at least to Bran (and possibly any reader with a fear of Dem Bones). Also, the fate of the painter in book three is literally this, since the Wild Magic the Greenwitch unleashes upon Trewissick is to resurrect the ghosts, dreams, and nightmares of the past so as to haunt the town, and they end up dragging the painter away in the place of a supposed traitor who had doomed the ship Lottery. (One wonders if Susan Cooper had read Shirley Jackson...)
The afanc attempts to be this for Jane, but once the Drews realize courtesy of Bran's display of power that all it is is noise and ugliness but little else, it ends up becoming an in-story example of Nightmare Retardant.
The Dark Is Rising is the second book in the 'Dark Is Rising Sequence'. The first, Over Sea, Under Stone, was written as a standalone children's story and published in 1965, and it took eight years for the Trope Namer to appear – during which time Susan Cooper retconned some of the first book's elements into aspects of a deeper story that then played out over four further volumes. Some fans argue the sequence can be read without the first installment; certainly The Dark Is Rising is the source of many of the series' defining features listed here, its best-known volume, and came to lend its name to the sequence as a whole.
It is quite possible to start reading from the second book of the sequence and miss out on Over Sea, Under Stone altogether. It leaves no obvious gaps in the story background, and there are no callbacks to events there. The only significant plot element coming out of it is the manner in which the scroll box ended up in the sea - something that isn't really dwelt on in Greenwitch. (Although references are made to Hastings, Mrs. Palk and her nephew, and of course Rufus, but none of these are critical or even give away plot points. The Black Rider later appears as Hastings to the Drews when they meet him again in the last book, but again nothing specifically from the first book is referenced.)
The death of Cafall, and even worse, the fact that Will literally cannot feel any sympathy or even empathy.
The drowning of the Lost Land.
John Rowlands denying his wife.
The ending of the series.
Deliberate Values Dissonance: The racist and his son in Silver on the Tree, targeting Pakistanis (and the child and family in question was actually from India). The fact his views and hatred are portrayed as being of the Dark, or at least the sort of thing that the Dark would promote and encourage, but at the same time Merriman later says that even without the Dark, "good men will still be killed by bad, or even other good men, and there will still be...anger and hate", does a very good job of relating mythic evil to the more everyday evil we find in the world. May create a great deal of discomfort in the reader thanks to the Values Dissonance, but that's precisely the point.
The Woobie: The Walker, for about the first half of The Dark Is Rising. Once it's revealed he's really Hawkin, and he chooses to ally himself again with the Dark, he becomes a Jerkass Woobie.
