There are 3 big areas of spending in the US, Medicare, Social Security, and Defense. These are the giant consumers in the budget (or expenses anyway, since of late we haven't bothered to pass any budgets). There's a 4th item that is fairly large (interest on the debt), but it's even less negotiable (downwards anyway) than the other 3---in fact it could become ruinous if interest rates rise to more historically normal levels.
The fundamental issue is that nobody has a real political incentive to fix the problem. Yeah, the Republicans might like to peel a little money off things like the Department of Education, NPR, or Planned Parenthood, but that's not where the real money is. If we actually wanted to solve the deficit/debt problem we'd have to significantly cut these big three. Now, who wants to do that? The Republicans know that they'll be blamed in the media for whatever happens, so they won't. The Democrats don't want to cut them either,because they know they'll be massacred in the polls if they do. The recipe is one for kicking the can down the road, and I think that's what we'll see after lots of posturing and blustering.