Even though online classes sometimes provide opportunities for student interactions such as message boards and emailing, it's almost impossible to get to know the classmates online as well as in a real classroom. In an online class, students have fewer opportunities to develop a relationship with their professor because they can't attend office hours or ask questions after class. However, online classrooms offer students a lot more independence than traditional ones, which is helpful for people with busy schedules.
Often times, students can take the online class at any time during the week. Thus, to be successful when taking online classes, a student needs to have a lot more motivation and self-discipline than he would need in a traditional classroom setting.
It is almost impossible to take classes that require a lot of laboratory work online. In most cases, the professor can show the students a video that demonstrates a concept or a process, or perform a demonstration using a webcam. Sometimes students can study on interactive websites, but this type of studying can't compare to the real experience of doing experiments in a lab when it comes down to chemistry, biology, various types of engineering and other hands-on sciences.