There is one question in the post 9/11 world that we still haven't effectively answered, and that is simply put; What kind of military do we want? It's not as simple to answer as you might think. Currently the Army is the be all do all, and is much like the Swiss Army Knife in organizational form. Many functions are available and while the Army can go pretty much anywhere and do pretty much anything, with sufficient support.
'Take Afghanistan. That is a landlocked country, that theoretically no army should be able to sustain operations in. Let alone for a full decade (and counting). The missions there range everything from standard infantry assaults, to civil affairs, counter insurgency, even the new mission "nation building" which is a nebulous task that might range from civil engineering to actual politics. The Army as an organization can do some of everything. Most Americans do not know or even understand the capacity of its military, or even what the elected leaders are ordering them to do.
For instance who do you think has the largest small boat navy in the world? Iran? China? Some corporate entity? Try the United States Army. That fact is so surprising (most people are surprised that the Army even HAS ships) that most people are baffled and at a loss to explain it. Would it surprise you that the Army still maintains the ability (if in a limited fashion) to conduct seaborne assaults? To prevent inter-service rivalry, this fact is not often touted, but the ability is still there. It's not just equipment though.
The American Soldier and Marine is expected to be able to adapt and overcome no matter what climate, environment, or situation they are dropped in. How does one train for the mountains of Afghanistan? We have very few peaks that can match their rugged terrain, and even so it would be impractical to run more than a few companies at a time through such training. We can train in Jungle, Desert, and certain winter environments, we can simulate many environments for tactical purposes, but the problem is that each require different tactics. How many tank battalions were there in Vietnam as opposed to say Desert Storm? You can run whole divisions across the desert, whereas, in the jungle you might have one or two tanks supporting light infantry battalions.
The problem is in this post-cold war world, war hasn't gone away. The Soviets aren't going to attack the Fulda Gap anytime soon, but that does not mean that we must not be prepared for a similar scenario. The Airborne hasn't made a true combat drop since Korea (Operation Junction City, was not a true combat jump, and the assault on Bashur Airbase hardly seems like a "combat drop" when there were Spec Ops teams sitting in lawn chairs on the DZ) but that does not mean that all six Airborne Brigades do not train as though they might.
The problem is that policy makers and bean counters alike don't like the budget it takes to have "the best trained fighting force in the world". For the military to prepare for a war they are often given direction by congress as to what enemy they should prepare to fight. More often than not the policy makers are wrong and the military has to figure it out after the war's already begun. Getting elected leaders to sit and talk about the threats they face is much like pulling teeth from an alligator. Even accepting that there might be a war will send certain politicians into a tizzy.
Our military is functioning on the inertia. As the decade changes with a massive debt on our back, we're going to have to ask what kind of military we want. The tanks and APCs of the Cold war are ill suited to the mountains of Afghanistan, and the insurgency warfare that has become a worldwide problem, but the light infantry does not have the same tactical maneuverability that those tanks would. You can specialize your tactics once you're actually fighting an enemy, but until you are you must be a generalist, and that costs money.