What one thing would you change about the Military?

I would change personnel management, make it more like the private sector.

EDIT: Based on the confusion generated by my initial comment pointed out by Cpl Dennis F., i would change the command-economy model of personnel management in the military in favor for a free market, like the civilian sector. I would empower commanders (at as low a level as possible) to manage their budgets and hire and fire- at least for billets and assignments, if not for the service itself.

Example: Lt Terrible is incompetent as a platoon commander. His Battalion Commander fires him and he goes into a generalized waiting pool where he applies for other jobs that a 2ndLt of his specialty can fill e.g. Div Intel Officer he eventually has to accept as less desirable position, or lower prestige, or maybe even lower pay. Conversely, Captain Awesome wants to stay a platoon commander, even though he's now a Captain. He is willing to accept a smaller paycheck and tells his Battalion commander this, and is allowed to keep doing the work he loves.

The article linked below fleshes out in great detail the theory that good, high performing members of the military depart mostly because they don't get to keep doing what they want, largely due to a lock step method of personnel management and development over which they, and their commanders, have virtually no control. The article was later parlayed into a book, also linked.

http://www.theatlantic.com/magazine/archive/2011/01/why-our-best-officers-are-leaving/308346/

http://www.amazon.com/Bleeding-Talent-Military-Mismanages-Revolution/dp/ [login to see]

The fact that it has become a social experiment.

TIS and TIG. There should be a way to "test" to the next pay grade. If I come into the Army at the ripe age of 32, I should be afforded the chance to test into a rank more appropriate for a man my age and knowledge. Instead, you have these 20 year old "Srg" who have no life experience, telling me how the best way to do something. If you try to help them and point out where they could do it better, they say," I'm the boss, I know better". The army could use people who have life experience so much better than the way they do.