The main problem with Arya is that the actress is 3-4 years older than the character in the book (as is the case with all the Stark children). This complicates how the character interacts with others - it just wouldn't work to have Maisie Williams with a nickname of "squab", or "weasel", as in the books. She has far too much presence for that. So they had to figure out something else to do with her. And the fans of the book are not going to like "going off Cannon".
Her age and apparent clarity of thought (in the HBO series) also create logical anomalies when she "stays on script" - making illogical decisions only an 8 year old would make.
We're just going to have to learn to deal with it - as the actress isn't likely to stop growing any time soon. It's possible that character arc may differ wildly from the source material by the end of season 4. Frankly, I don't see how it could not.