The one criticism that seems to have stuck on Chuck Hagel is the charge that his views were outside the "mainstream" consensus arrived at by that bastion of foreign policy understanding, the U.S. Congress.
This consensus is treated as if it simply reflects a self-evident reality about U.S. interests, or portrayed as the result of careful, bipartisan deliberation. The Hagel hearings exploded that myth.
Instead, we saw first hand that the bipartisan consensus is sustained because policymakers with career ambitions can't really afford to deviate from it without risk to their career. It's an incentive structure that selects for conformity. Hagel's "safe" approach to the hearing, where he choose to mostly elide (or bumble) tough questions rather than address his critics head on, will only entrench this dynamic. Even if he survives the confirmation process, any current or future foreign policy wonk with high career ambitions who watched the Hagel hearings must have absorbed the message: much better to play it safe.
Update: Larison adds more:
One thing I would add to this is that the conformity that is being demanded of politicians and officials doesn’t have much of anything to do with an informed understanding of the relevant issues. As Scott pointed out in his post last week, adhering to the consensus view seems to require flat-out rejection of weighing different policy options. It doesn’t seem to matter whether a person has reached the “right” conclusion at the end of his deliberation. Evidently, the process of reaching that conclusion must not involve entertaining or considering anything besides the “right” answer, and if it has that becomes grounds for suspicion. It’s not just that the consensus view isn’t the product of careful deliberation, but that careful deliberation itself is taken as a sign of possibly unacceptable deviationism.