In a traditional classroom setting, you have your professor and your friends to remind you of classes, pending assignments and upcoming exams. In an online learning environment, you need to do everything yourself. There are no friends to motivate you or professors to guide you emotionally when you are down. It becomes very easy to give in to procrastination and disorganization in such an environment.
This can be hard, especially if you are an outgoing person who likes to be surrounded by people. An online learning environment has instructors and fellow students, but all virtual. You may talk to them over voice equipment and even see them through webcams, but such an experience may not compare with the level of human contact that comes with a traditional classroom setting. You may feel lonely with only your computer to give you company.
If you loved those traditional classroom sessions where a single question sparked a lively debate among students, then you may be in for a disappointment here. You can send questions through emails or post messages and even debate via instant messengers, but you may miss the presence of an instructor in front of you and the livewire communication that you experience in a traditional classroom.
Some employers may not accept online learning as a valid mode of study. This may be because of the belief of some employers that online system of learning does not impart the same level of expertise as a traditional classroom education does.