* A stack overflow for specific questions

* The choice between video OR text for lectures

* a place for people to share their notes, mindmaps

And finally, one of the things I miss most in something like Khan academy: A Syllabus. A tree showing where branches of knowledge begin and where they end.

I know Khan academy try this via the knowledge tree (or whatever it is called) but I find that to be a big mess. They do many great things, but that thing isn't one of them.