Sometimes, if you already know what major and what school you'd like to transfer to, you will be required to take certain classes as part of your general education classes. Often times, the same class could have a different name between the two colleges, so figuring out which class is the equivalent can be hard.
Even if you think you have taken the equivalent course, sometimes transfer students find themselves feeling a bit behind, especially if one of the classes they took at their community college was a prerequisite for a class at their new college. This can pose as a serious problem for the transfer student since she will have to do a lot of catching up. However, sometimes, the transfer student does not always have time to catch up and may have to end up dropping the class and having to essentially retake a class and try the dropped class again.
When you enter any college you will have to take a placement test for both English and math. If you have to take remedial classes, this may prove to be an issue in transferring. because there can be more than one remedial class, if you ever need to retake a class you may be forced to stay an extra semester.
Many schools have a test that you have to take as a requirement to graduate. San Francisco State University, for example, once required students to take a writing proficiency test. Because not every university requires such a test in order to graduate, community colleges have no way of knowing that they will need to prepare their students specifically for a certain kind of test. If students did not go to a community college first, every English class leading up to the proficiency test would be taught with the goal in mind to be prepared for that test. Students who did not pass the test were required to take a remedial class.