I have found that time is a significant constraint. Just because it's online and you don't have to physically go to a classroom at a particular time doesn't mean that there are no time constraints. The idea that online learning is much more flexible in terms of time is often overstated. The reality is that you still need to make time to get on the computer to access materials, do the readings, keep up with the online discussion board, etc... Depending on how the online course is designed, you may end up spending more time on the course than you would be if you were attending a face-to-face course. Online course designers need to make sure they pay attention to how much online and offline time their course will require.
Most people who dropped out of classes I offered online did so because they didn't have time to fully participate.... (at least that's what they said) -- only one said that she just didn't like the online format and would rather attend a face-to-face class.