I don't think it's the subtitles -- I think the plot of the movie just has a gaping hole in it, which is not adequately papered over by some fast action sequences with no parallel in the book.

The movie is charming and delightful, as all Studio Ghibli movies are, but it suffers from Miyazaki's inability to deal with a proper villain (most of his movies have antagonists with understandable motives; by comparison, Diana Wynne Jones pretty much does Villains; believable, but thoroughly nasty and unredeemable). The novel did have a villain, and the villain drove the main plot; the movie doesn't, and it also removes a parallel-universe aspect of the setting, so that the plot gets distinctly soggy towards the end, when it's heading towards a resolution that the preceding story can't really provide. I think Miyazaki does better when he's using his own material than when he adapts other people's.