Accepting kingship - I think he had a little choice in that. At the time Joffrey was legal heir to the Iron Throne but he was unacceptable for the North.
Sending only Theon to treat with his father - I think that even Ned wouldn't be that naive. A bad move.
His won all his battles but lost the North.
Beheading Kastark - he should have allowed him to take the black.
Marrying Jeyne - stupid, stupid, stupid thing to do. The only other person I can see doing something similar is Sansa as she was in AGoT.
Probably naming Jon his heir - he was in difficult situation, and was right to think that nor child nor some distant cousin in Vale would do, but he may weaken position of NW forever.
Overall, it doesn't look so good, but I have to say that he was one of the more competent kings (more able than Joffrey, regent Cersei, Robert, Balon, but probably worse than Renly, Stannis or Mance, truthfully I would put him in the same league as Dany) he was not a great one.
Edited by lazy turtle, 24 February 2012 - 09:01 AM.