Such prose is common to all of James's novels (or at least, all the ones that I have read; the introductions to these assure me that he doesn't dramatically alter his style elsewhere); what appears to make 'What Maisie Knew' stand out is James's masterful deployment of his titular heroine. Maisie is an ironic construction of genius: by presenting her parent's sordid personal lives through her innocent point of view, James allows a great deal of ambiguity, and a great deal of humour, into his work. Furthermore, Maisie's almost angelic perfection provides a counterpoint to the adult characters, and is, perhaps, necessary to prevent the story from becoming some cheap morality tale: Maisie's desire to please everyone, and ability to forgive everyone, can often prove divisive, but also prevents James from making any crude moral judgements.
Maisie, as a fictional construction, has some flaws, however. Her passivity throughout the work, while it may be one of her virtues within James's conception, seems to be almost to blame for the plot's stagnation: she must choose who, from a wide assortment of unsuitable adults, she wishes to live with. That this decision is set up within the first thirty pages, but delayed until right at the end of the novel, means that the body drags and suffers from a lack of purpose. This is exacerbated by James's desire to mirror every scene, giving the novel a symmetrical feel: while this often highlights the similarities and differences between the characters, and can be frequently amusing, it engenders repetition in terms of narrative. In short, the book is over long, and lacks a certain compulsiveness which I would have appreciated. Maisie's perfection also seems to imply that no adult guardian is necessary for her, and, indeed, that one might be detrimental for her, hence invalidating, to an extent, the whole plot. Perhaps James does this intentionally; I'd certainly like to imagine that the novel attempts to critique the polemic it could well have become in less adept hands.