After an actual desire to learn (which several others here have mentioned), the biggest obstacle to learning is bad teachers. I don't want my teacher to be a vast self-curated backwater of internet tutorials and apps. I don't even want options to customize my learning. I want an opinionated, sassy domain expert who inspires me, pushes me, and clearly delights in the subject.

Examples: http://www.staff.science.uu.nl/~hooft101/theorist.html (thanks tokenadult) http://www.thegreatcourses.com/tgc/courses/course_detail.asp... http://how-to-learn-any-language.com/e/index.html

If you had a learning website built around teachers rather than subjects, I would definitely check that out.