Jaqen, Jon is the proto-typical young, humble, semi-reluctant hero predestined to save the universe that I've seen a million times before. He's been done to death already. Frodo Baggins, Harry Potter, Shea Ohmsford, Belgarion, Pug/Melamber, Rand al'Thor, Tuck Underbank, Taran Assistant Pig-Keeper, Prince Caspian, Paul/Muad'Dib/Usul, Lord Kalvan, John Carter and many others fall into this category. Other hallmarks of this character are friends who are loyal to their deaths, beautiful women who see through the heroe's self-doubt, his assuming leadership at an absurdly young age, and an arch-villian who is blinded to the heroe's weaknesses. Jon, to me, is a walking cliche. Now his friendship with Tyrion, his meeting of Aemon Targaryen, his attempt to run away and his dalliance with Ygritte are the four interesting things about Jon. Qhorin's execution was predictable, the continual slaps on the wrists he receives from Benjen, Jeor, Qhorin, Donal, and Alliser are laughable. Catelyn's spite was beyond obvious as a means of generating sympathy for him (I'm not saying it did not work, 'cuz it did... just saying it was obvious). On the other hand, his POV really introduces Tyrion, the Wall, the Night's Watch and it's issues, the Wildlings, the disastrous Great Ranging, the Others, Maester Aemon, Dolorous Edd, and Mance's assault. Jon's POV is exciting, he is not.
Regarding Arya... even in fantasy, how many short, skinny, nine year old girls have killed eight or nine men? Sure she lives in extraordinary times and has been in numerous impossible bloody situations, but really... come on, throw me a bone here. Arya has become a pre-teen mass murderer... Just let that sink in. Now think, is she still somewhat normal? Is she psychologically functional? Yes she is... and I don't think she'll develop from here on out normally... In fact, I'm flabbergasted that she can even function at all. In my opinion, her rage should come through as her default emotion. I'd love to have a specialist in Child Psychology read Arya's POV's and give us some feedback. I think she will never throw off the yoke of the Faceless Men. She'll always be a stone cold killer. She'll never be able to undo all the years of murdering to survive and become Lady Stark.
For the record, I don't like Jon, but I do like Arya... I'm just not overly fascinated with her. I don't find myself wondering what will happen to Arya as much as I wonder what information I'll be able to glean from her POV.
Just like Jon's POV's, I love the story that Arya's POV's tell. From her perspective we really get to the nitty gritty of the war and it's effects upon the inhabitants. Rape, murder, rage, revenge, barbarism, and sadism are shown as the norm rather than gold crowns, white cloaks, feasts, and stirring speeches. We glimpse two of Robert's bastards, we continue to know the Hound, we are introduced to Jaqen and the Faceless Men, and we view the willfulness of Lyanna mirrored in Arya.