It's been well noted that one of the biggest problems soon to be (in only a matter of hours now) President Barack Obama will face is the incredibly high expectations that people have for him. That being said, I think it is also easy to go the other way and to think that nothing will really change. Personally, I'm optimistic that he'll be able to get some good things done but I also realize that some of the expectations that people have are not entirely realistic.
The United States certainly has some very big problems that are not soon to go away. The U.S federal debt is one of those factors and in the current economic problems, it is not likely that major spending cuts or tax increases will happen anytime soon.
Another problem is that the President just doesn't have the power to do some of the things that need to be done. Congress needs to do what it is responsible for and many other problems occur within various agencies and within states. An example of that would be the problems in California. Years and years of ballot measures have tied the hands of the state's legislature and the governator (Arnold Schwarzenegger) himself has been struggling to get action to take on that state's approaching bankruptcy.
The U.S has not been well governed for some time time. I don't think it was really well governed during the Clinton years (though again some of the blame should be placed on Congress and State governments) and under Bush it was much worse.
The one area (and a very important area at that) where we are likely to see significant change is in the U.S foreign policy. The President has much more control over that and a new U.S approach will go a long way in winning over the hearts and minds of the world's people. Under Clinton, the U.S disengaged from much of the world (clearly evident here in Asia) and when the U.S did reengage under Bush, it was through invasion and coercion.
China has benefited greatly from what Joshua Kurlantzick refers to as a decline in U.S soft power in the region. China's system of government and corporatism allows it to make rapid decisions and take rapid actions that the U.S system simply doesn't allow for. Everyone knows that the U.S moves much more slowly than China, but many in Asia also fear a Chinese hegemony. A think a lot of people would start to warm back up to the U.S if the U.S actually dropped the "war on terror" bit. Think of it, if you are a government official in Cambodia and every meeting that you have with the U.S results in a request for your country to participate in some way, in some 'war on terror" that is nearly irrelevant to your country's problems, meanwhile there is a professional Chinese diplomat just down the hall who is ready to make trade deals and offer aid, who do you think you'd rather talk to? Add on top of that the fact that Chinese officials seem to know their jobs more than their American counterparts, and it's no mystery why China's image is far more positive than the U.S image in so many Asian countries.
Now, I don't mean to suggest that the U.S has become irrelevant. It most certainly has not, but it really has fallen far in just a decade and a half. Lots of Pew research that backs that up, but I'm too lazy to dig it up right now.
So, in the foreign policy realm I think we should expect a major change with Barack Obama. But, on the domestic side there will be a great deal of resistance from some quarters and we should think more of this being the beginning of a long road rather than the destination.