He plays a stick-in-the-mud like Ned, with less fire than Robb has. To that, yeah, Harrington does a great job, but it's not exactly an energetic and demanding role. I'm not underselling it, and I like Jon as a character in the books to begin with, but the actor does very little to win me over.
In conversations with people I know (friends, my local comic store owner, and one of his friends, primarily), I formed a list of who I considered the best castings and portrayals for the series as a whole. It was meant to be a top five in no particular order, but after the four I came up with, I didn't really care for a fifth.
Again, in no particular order: Nikolaj Coster-Waldau as Jaime Lannister, Iain Glen as Jorah Mormont, Aiden Gillen as Petyr Baelish, and Richard Madden as Robb Stark.