About halfway through my college experience, I switched gears from extreme attendance-and-attentive-note-taking to extreme self-teaching, out of necessity (I was taking too many classes and working simultaneously, so I didn't have time to attend each lecture each week). Of course, I still did the homework assignments, studied for exams, etc.
Except for the occasional "gotchas" where professors say something random in class that shows up on the exam, it didn't adversely impact my GPA, and it saved a ton of time. I think I learned more efficiently on my own, which led to saving a ton of time shuffling from physical location to physical location. It freed up more time for working, exercising, and sleeping.
Given this experience, in retrospect, I think the most valuable parts of my education were not necessarily the classroom, but rather:
50%: The (well-written) syllabi. They point out which resources (books, blogs, websites, etc) are worth your time, if you have an interest in the subject. The best ones are filled with opinions, and "optional resources if you want to dive deeper into X". Even the lazily-constructed ones are a great foundation if you have no idea about how to begin learning X.
25%: The classmates and TAs. Oral/written feedback, delivered efficiently and with empathy is not essential, but a great catalyst to development. This one's a bit personality-dependent, for other's it might be higher or lower.
25%: The assignments, projects, and deadline pressure. To learn things, you really need time-spaced practice. Reading is not sufficient. The deadline and grade pressures erode as you get better, but they're invaluable early on.
I think that you can get 90% of each of these online, just fine. But that's me! It might require personal conversation for others, but in my experience a good collection of reading and tasks, some form of IRC, and rudimentary deadline pressure are more than sufficient to replace the classroom for self-starting individuals. I think Coursera's relative success proven that case.