The characters were plausible: they were frightened, and inconsiderate, and content in a way that made for a fully-fleshed person. Even though I knew what happened in the story, the situations were still tense.
Something I may not have appreciated so much when I was young, were the wonderful descriptions. For the most part it's set in Scotland and she would often start a chapter with a sentence or two describing where they were. And, perhaps because I've visited Scotland, I could visualise exactly the scenery she sketched. Reading up about her, I find that she was an artist - specialising in miniatures - and her use of colour in the story was very evocative:
the distant lift of damson-dark uplands that showed through a break in the oak woods.
A little chill wind came soughing across the garden, silvering the long grass,
the short hill-turf that spread here and there like green runnels among the bell-heather
As a child I read every one of her books I could find, either in the library or bookshops, and I'm tempted to start searching for them again.