Heh.
Honestly people tend to look at too few things to talk about why things have been changing and going downhill. For one point, I don't think things on the whole are really going down hill all that much (at least compared to what most tend to assume).
The biggest problem facing the US right now is actually rather interesting. Effectively our government doesn't match our culture anymore and thus is having pains.
You see, our type of government is very much geared to a British mercantile culture. When our nation was founded that is exactly what we were, and in fact we remained a mercantile culture up until the 70's or 80s. Around then however we made a switch over to a service economy, and our democracy has suffered as a result.
This is particularly a problem with the US because the founding documents of the nation are actually revered almost to religious levels. You will not hear someone say "Down with the Constitution, long live the United States" EVER because it is just politically and culturally impossible to do so without being branded an outcast. By contrast in a European nation like France such sentiments are expressed from time to time without the extreme reaction you see in the US. As a result it is very difficult for us to break from the initial ideas of the nation's founding in order to evolve our government to meet the current challenges (this on top of a government system that is already inefficient by design).
There are of course other factors involved as well. The large divide that currently exists between the two political parties while they both ironically agree on the large majority of issues for instance. Both parties are completely convinced they are defending the true heart of what the US is, and both are convinced that the opposing group is taking actions which are going to undermine the nation completely, and are utterly unable to bring themselves to find a middle ground. People complain about how much the Republicans are using filibusters currently and how they are using them at the highest levels ever, but they fail to mention that previously it was the democrats doing the exact same thing while Bush was in office and back then the Democrats were at the highest levels ever, and before them it was the Republicans again, and before them the Democrats, and so forth. Each time they reach new highs of how often it is used, and when the tables are turned the other party goes to even greater extremes.
Another matter is that political savvy in the US (and actually all modernized nations) has declined sharply with the introduction of TV. The prime example of this is the Nixon/JFK debates where those who heard it on radio or read the transcript thought that Nixon had won quite clearly and those who saw it on TV thought JFK had won. Ever since TV was introduced it has ceased to matter what you say so much as what people see while you say it. This has even caused a huge problem with news reporting, and is maybe one of the major reasons that current news agencies are finding such problems competing in the internet age (where people are quickly finding they get more accurate information from a few quick web searches and a bit of reading than from watching the 9PM news).
Blah, long winded again.