..But You Get to Schedule Them
Probably the biggest change between high school and college is getting to decide when to take classes. Not a morning person? Start your classes at noon! While there are definitely limits - many classes will only have one time slot available - classes that many people need to take, such as freshman Composition, often offer multiple sections, allowing you to choose the one that best meets your needs.
Another change is getting to choose how many classes you want to take. While the number of credits required for full-time status varies from school to school, generally a full load is fifteen semester hours, and most undergraduates take between twelve and sixteen. Most classes are three or four credits, with science classes that have labs often being five; as such, a full load is generally four or five classes. A quarter is 2/3 of a semester, so if your school uses quarters rather than semesters, multiply the above numbers by 1.5; 4 semester hours would thus be equivalent to 6 quarter hours.
What many people have trouble with is the amount of responsibility that college students are expected to take for their own education. Teachers generally won't try to track you down if you don't show up for class, and the homework to class time ratio is a lot higher; classes generally meet two or three times per week, and students are expected to spend several hours studying for every hour spent in class. Additionally, many classes (aside from math) don't have regular homework assignments, which means the few assignments they do have are that much more important to the final grade; in some classes, the exams are the entire grade! Classes often have strict prerequisites, so failing a class means pushing back taking an entire string of classes. Additionally, courses in your major generally require at least a C to pass. Expectations get higher as your progress. For graduate students, a C is a failing grade!