Gender is the one social division that is universal in human societies, and is for most people deeply connected to their most important (i.e., intimate) relationships. It's also among the hardest characteristics to alter, which is why changing genders it's such a common theme in literature. The differences in physical strength are also important. There are many things men and women are roughly equal at doing, including just about everything in a modern middle-class society, but take any man and any woman between 18 and 60 and reasonably healthy, and I would bet that at least 9 times out of ten the man could beat up the woman. That forms a serious power differential, especially when there are so many men (even if it's just a small percentage) who desire to harm women.
These are all things that more or less cut across cultures. Of course age is another, and that's also a big identifier -- but it's also something everyone expects will change, and one that society at least gives some preparation for changing. Class? That is much more easily and more often changed than gender, but still hugely important, which is why rags to riches stories (or riches to rags, or rags to riches to rags) are also major themes of literature.
I think culture shock, as suggested by findingreason, is one of the most apt desccriptions of the transition experience, both before and after transition.