This is a very mature question. I am sure that there will be many differences of opinion, but here is what I think.
First, the economy is not very strong. In light of this, I think one of the greatest problems is that there is not enough jobs. The social implications of this are enormous. Without jobs, people get discontent as they have too much time on their hands and little work to do. In addition, America was built on the premise that hard work would pay off and that anyone who wanted more opportunities could get them. If this changes, our culture will change for the worse.
Second, I also believe that the environment is another huge problem. If we do not make changes to the environment, then we will have big problems in the future. Think of the air quality in China; it is not very good. If America became this way, it would cause so much health issues.