It seems like the Arya storyline ended up on the backburner. They kind of forgot about it and weren't sure what to do with it, and they just sort of abruptly ended it. It didn't seem like she learned much or anything really came out of it. The quality of the writing has taken a noticeable dip this season. It's still better than most shows, but you can tell they're just winging it at this point because they are out of books to base the show on.