SO is great for finding specific things. If you understand fundamentals it's easy to port your knowledge to some other framework by just googling, and you'll end up on the right SO question. What's the syntax for making a dictionary in some language? What do they call the dictionary in that language (map maybe)? How does the && work in the new c++? And so on.

It's much harder to learn stuff that isn't specific on SO. Things like working practices, what kinds of architecture are out there, and questions where what you're after is a kind of survey.

One thing I don't get is the speed with which the top answerers can get back to you. I've personally had questions answered by Hans Passant, Jon Skeet, Marc Gravell, Martijn Pieters, and probably a few others off that top list. Sometimes it's within minutes. Is their daily life substantially to answer questions about the technologies they are associated with? I work all day as a coder, and I don't have time to sit there and type out answers.