Very interesting subject.
If we look back on history, as that is the only source of information to base assesments of the future from, no "super power" has ever remained a "super power" (be it military, economical or cultural).
If history repeats it self, as it always has, the US will not be the worlds leading economy and worlds leading military super power in a long term future. When that change happens and what country will take its place I cant say.
There are a few things that can be made to prolong/lessen the fall of US as the leading nation.
One thing has been mentioned, education. US education is very segregated atm. US has some of the best schools and some of the worst education in the western world. The "elite" is very good, the avg is not very good.
Complete oposite to sweden, where Im from. Here we have extreamly good avg level of education but the elite level (for lack of better words) is not as good as in the US.
We have free college education which means that everyone has an equal opportunity to a degree but at the same time it means that the top schools dont have the resources to compete with top international schools. While our top colleges are good on a european level they arnt "elite".
Education is always key to a countries evolution (yes it is constant evolution).
US has a definate problem in its education system as way too many americans have too low level of education.
Another problem the US has to deal with is crim. Crim levels in the US are very high and need to be contained and lowered.
Next problem is healthcare. Physical and mental health of a countries labour force is crusial to the long term sucess of a country.
Millitary expences is another problem. Im not in a mood to discuss US foreign politics in this thread but when spendings on the millitary put a nation in debt there is a problem.
Budget deficit means less budget for education, healthcare, crime containment and infrastructure. But it also means less cashflow from public sector to private sector (except for millitary industry). This in turn hurts the revenue of the "civilian" companies in the nation and hence slows down the economy. This resulting in lower exchange rates for the countries currancy.
A low exchange rate on the US dollar makes US companies much more exposed for international takeovers. International takeovers on US companies in the long term means less security to the US economy.
Siemens buying IBM and moving research, development and production to a baltic state isnt as far fetched as you might think. Especially with the USD nearing a all time low (not all time but modern days).
Considering this you americans really should appricate what you have and be HUMBLE about what you have. The more humble you is the less likely other people are to feel the need to take it away from you.
Tex