Well, I'd tend to start with the premise the United States is going to go from the world's only superpower to the leader of one of five main fairly equal spheres of economic, political, and military influence in the world. That's transition is visibly beginning. There are a lot of people who really study these things writing books and so forth on this topic, but I don't think you need to be a scholar to see that we're declining in some ways, while the European Union, Russia, and China are on the rise. The Euro is becoming the international currency of choice, for example, and it's only a ,matter of time before the British pound is abolished and incorporated into it, which should add to the Euros long-term strength and stability.
America's future is going to depend in part on whether or not we are adequately able to adjust to these changes from a public policy perspective. One thing that's going to become more important is diplomacy -- we're going to need the world's cooperation more than ever to accomplish our military and national security goals, because we won't have the power or strategic influence to do it ourselves or mostly by ourselves. The sun is setting on as a defacto world empire, now we're going to need to act in concert with other powers. Not to seems self-serving as an Obama supporter, but future Presidents are going to probably have to be more in his mold than the Bush-McCain mold by virtual necessity at some point in order for us to maximize our strategic influence on the future direction of the human race.
We also should probably in the long-run consider closer political and economic ties to Canada, a sort of North American Union. That would help curb some elements of our overall strategic decline. Mexico unfortunately is not in good enough shape to consider doing something similar with right now, their economy is in tatters and crime and corruption are so rampent that people are rioting over it as we speak.
Also, when we look at the US GDP as a percentage of the world economy (our slice of the economic pie) go down, economic inequalities between families (and individuals) are likely to become more significant in terms of their impact on daily life even if the gap between rich and poor doesn't actually widen. Let's face it, if the dollar has a bit less purchasing power, the rich are still going to be rich and won't have to necessary make huge lifestyle adjustments, but the poor and middle class will. We're already seeing the beginnings of this in terms of gasoline prices lessening people's abilities to do things they are acustomed to like vacations and so forth, and in people being priced out of health care.
So, one thing that's going to be of vital importance is a domestic public policy adjustment on issues like health care and so forth, simply because whereas a couple decades ago, only a thin slice of people might be unable to afford health care coverage, that slice is going to rise and rise and rise if nothing is done. And that's going to be a key consideration I think, when the average person says "Were the 20th century our golden years as a country? Or are we still doing okay?". "Can my baby go see a doctor? Can I see a doctor?" and so forth are going to be one of the questions people will ask themselves.
We also might consider publically subsidizing universal broadband connectivity in some way. There's a proposal being considered by the FCC that would auction off a portion of the wireless spectrum (which is a government asset as a matter of law, the public owns the airways) on the condition that it be used to offer free advertisement supported wifi to 95% of the country within 10 years. That may seem minor now, since a majority of folks can afford the Internet, but again as our role in the world economy changes, the Internet may become less affordable, while at the same time more and more people consider the Internet their primary source of entertainment and information (television is in a relative decline, though I don't think it will completely disappear). So, in essence, people may have less access to something they increasing view as more important as time passes, if we don't take measures in advance to ensure that the Internet is universally accessible. And that's going to make a big impact on whether people subjectively think our best days are behind us ("Do my friends and I have access to the chief form of national entertainment and information?").
Finally, I think a measure of people's happiness with our nation's happiness ("Are our best days behind us?"), is going to depend on how the average American psychologically adjusts to a world where we are not the alpha dog. Could we handle, for example, half of our movies and entertainment coming from places like Europe? I don't mind at all, Dr. Who is one of my favorite shows, but it is an open question how the average will react. Will that have a psychologically depressing effect on most people or will they say "Who cares? We're all people".
Actually, that last night might be the key one in terms of preparing Americans to psychologically accept what is inevitably coming in terms of no longer being the world's lone superpower. We may have to, while still taking pride in our country, begin thinking of ourselves as firstly members of the human race and citizens of the world and secondly as Americans. I think a lot of Europeans and so forth already think of themselves that way. It's not a moral judgment on my part saying that that is a better or worse way to think, it's rather a practical judgment that people who think of themselves as citizens of the world first are going to be happier in a few decades than people who see themselves as Americans set against the world, because while America may decline, as a whole the entire world is likely to move forward. How we psychologically adjust is also going to impact whether we elect leaders who can cope and make the adjustments the country needs to make to come through this thing in good shape.
It's also going to make an influence in terms of how much freedom we have. Leadership that sees what's coming as an opportunity will likely allow us greater freedoms versus leadership that sees what's coming as completely unacceptable and is deeply suspicious of these other 3-4 powers and puts us on a constant warfooting. A constant war footing generally means less freedom. And, that, too effects the mood and the perspective of the average person.
So, when we ask ourselves, our are best days behind us, the answer may depend less on circumstances than on a vital followup: Who are we? Are we first Americans or first human beings?