Part of it is how compelling the material is, how true it has been to the books (overall), and how good Maise Williams is; she's the most impressive child actor I've seen in a long time. I've always said if done right, her arc in ACOK could be the best thing on television. It started a tad shaky but has rebounded significantly.
The Greyjoy stuff is also very focused and impressive, as is Tyrion's stuff. Whereas so far the Robb stuff seems pretty inconsistent and scattered.