No one ever really talks about reverse culture shock, so it is usually rather unexpected. And it is hard to imagine how difficult reverse culture shock will be when you are away from home and would give anything to eat your favorite American foods and listen to your favorite radio station. But reverse culture shock is a very real and very common thing and in some ways, it is even harder than going abroad. I heard the British Volunteer Service Organization (like a British Peace Corps) actually provides counseling services to service members when they return from China.
Reverse culture shock comes, in part, from expecting your home country to be the same as you left it. In many ways, the home country does stay the same, but being abroad for a long period of time does change the way a person looks at the world. Upon the return "home," everything is seen through new eyes and it is often hard to feel as comfortable as you did when you left. One of my first experiences upon returning home was being driven by a corporation with a huge lawn and all I could think was that a village of 200 people could subsist off of it.
I found culture shock to be depressing and distancing. It is hard to try and explain your experiences to other people and it is hard to readjust from a different culture. I felt like I was more an observer of American culture than a participant of it. I was able to appreciate different things about American culture and recognize how cohesive some aspects of American culture are, but I didn't feel like I was part of it all. Even following English conversations was hard when there were other English conversations in the background, like at a restaurant or with a TV on.
Of course, people adjust and culture shock fades. Different people have different levels of culture shock. Some of the people I knew had virtually no reverse culture shock and others had effects that slowly faded over 6 months. As the shock fades, comfort and familiarity return, and people finally feel like they are home.