We can see the beginnings of Rowling's authorial failings (and a hint of her strengths as well). She adopts Rouald Dahl's 'awful family' trope, though it's clear that Rowling does not have the gift of bizarre characterization or the knowledge of the darker parts of the human soul that made his books resonate.
She writes sympathetic characters, but not unusual ones. Overall her writing has relatively little character or style. Then again, mass success often requires leaving the more unusual elements behind. So she relies on standard character types, managing to keep them afloat through the patented perpetual plot of the airplane book.
She also pulls from that old British tradition of 'children lost in fairyland', seen often in early fantasy (Dunsany, Eddison), which Lewis also made use of. She also has the vast, unknown underground of magic just beneath our world which keeps itself always mysterious and quiet, much favored by Gaiman and other Urban Fantasy authors (though Rowling's invented world is strained and piecemeal, moreso as the series goes on).
The strength of the book is that it combines the tradition of the 'child in fairyland' with another British standby: the boarding school bildungsroman. It's the same neat trick Mervyn Peake pulled in 'Gormenghast', though Rowling's version is tame in comparison. Her tale of the intellectual, emotional, and physical growth of the young, outcast everyman is rather predictable, except for some insight into angst in the fifth book.
Rowling's prose is quick and simple, but sometimes awkward and without music or joy. It is not the sort of deliberate simplicity Carroll achieved by expressing complex ideas in playful terms. It is rather the sign of an author whose unsophisticated voice prevents her language from vaulting higher.
Simplistic elegance is deceptively difficult to achieve, and so it's hard to blame Rowling too much when she falters. It's unfortunate that she didn't put a few failed books under her belt before finding success, as such early outings are often best winnowed chaff.
Her plotting--as ever--is scattered and convenient; though in a shorter book, it shows less. Her plot twists, as usual, disappoint; they are not built upon progression of events but upon reader expectation and emotional red herrings.
It's the beginning of an enjoyable series, but there's really no need to start any earlier than the third book, when Rowling finally finds her pace and begins to lean more heavily on that which she does well, which helps to hide her faults. Watch the movie if you need a primer.
My Fantasy Book Suggestions