Megan McArdle talks military pensions, and focuses on the Commissioned side of the house. Which isn't a bad thing, per se. I agree with what she's saying, but I think she's missing the point.
If you want people to stay in a system where their lives are at risk, where they are governed in what they can or cannot say politically, if they must suffer under leadership both good and bad and deal with both in the same manner, to live a life where most every aspect of their lives is governed by some regulation, you're going to have to provide more than a 401k.
Try to foist off a civilian retirement on me, I'm going to act like a civilian. Which means that the next time some toxic leader tries to go off on me for cleaning up his screw-up, I'm going to tell him to go fuck himself. What the hell do I care? I'm getting my 401k!
"Soldier, take that hill!"
"Fuck you, I'm outta here! I don't need a 401k!"
Military pensions are different from civilian retirement because the military life is different from civilian life. The military gives up a hell of a lot, and many of them stay in because of that retirement. Remove that incentive, and you remove the incentive for good people to stay in the military.
Which, in hind-sight, might be why Obama wants to do this so badly.