I don't buy it. He's spent the entire time as Hand finding out just how dangerous the Lannisters are -- how they've schemed their way into power, how they've frightened Stannis Baratheon and Lysa Tully away from court, how they'd possibly poisoned the previous Hand of the King, how they've arranged to mask bastard children as the children of the king. Perhaps Ned wasn't imaginative enough to imagine Cersei as anything but a paper lion, but it's precisely the point that he relied too much on officialdom and title and not enough on actual power that marks his true faults as Hand. Tyrion grasped what Ned Stark never did, despite given ample opportunity. Varys did try to make him wise to the ways of the court, but Ned was too stiff-necked to see it. He didn't see the world as the way it was, but just the way he wished it would be. You cannot reshape the world by stubbornness or willpower. Ned failed to grasp reality. He was a kid playing an adult's game. He was honest and honorable, and I am not one to ever disparage those qualities (unlike other characters in the series who might have no use for those virtues), but you can be honest and honorable while still realizing what's going on. I don't blame him for necessarily failing to grasp the threat that Cersei presented, but his failings are more glaring than all that: he systematically failed to understand King's Landing, and that led to his downfall. And the reason for that was not ineptitude. Ned was not a stupid man, and he could grasp things quickly when he let himself. The problem is that he was unwilling to let himself see. Cersei rightly saw Ned as the greatest threat to her power over the king -- it's likely that only he could have salvaged things without causing a war, if he had only been wiser and less headstrong.