This is an excellent question and there will be many different opinions. However, I would have to say that the biggest problem that the United States faced from 1974-1999 was the economy. The reason why I think the economy was the biggest issue is because the economic situation today is a largely due to the fiscal policies of this time. Let me list a few problems.
In the 1970's the United States faces several large oil crises. As you probably know that the United States already hit it oil production peak a while ago. Now we are dependent on foreign oil. When Iran went through problems in the 1970s, oil prices skyrocketed. Sounds familiar today as the same thing is happening. There is an oil shortage and we have done little to solve it.
Second, during the 1987-2006, Alan Greenspan was the federal chairman and he propped up the economy falsely by credit. He had artificially low interest rates and it seemed like the economy was doing well, but in the end it made the United States into the greatest debtor nation in the world. We are living with this problem now.
Third, all the while as our economy was getting weaker or as we were setting ourselves up to become weaker, during this time we were also loosing our favor with the rest of the world in terms of an international image.
As I tried to say in post #1, our collective efforts to deal with poverty may have caused more problems than they fixed.
Housing for low income people is another problem area. From the Carter Administration to the Bush Administration, the Federal Government promoted subprime loans, which means it encouraged banks to make loans to less than credit worthy home buyers. That was not a problem as long as housing prices continued to rise, but when housing prices began to fall, people who could not afford their homes defaulted on the loans causing probably the worst financial crisis since the early 1930's. The number of people who owned homes increased a little bit, but one must wonder if it was worth all the damage that subprime loans caused to everyone else.
Poverty might be an area to research. As I understand, the poverty rate in the Truman Administration was one in three. At the beginning of the Nixon Administration it was down to one in six. Since that time the rate has not gotten better. One might wonder if the antipoverty programs (which became law during the Johnson administration, but which probably did not begin until the Nixon Adminstration) made poverty worse instead of better.