I think it depends on what you want _out_ of the system. For many, they want people with "ready job skills", which can be a challenge to give learners (budgets,

I think it depends on what you want _out_ of the system. For many, they want people with "ready job skills", which can be a challenge to give learners (budgets, always moving targets, etc). If you want to build good theoretical understanding and critical thinking, that requires the learners to work a lot, and is harder to quantify and test, but allows you to keep the material consistent over a long period. Also, different subjects can lend themselves better (or worse) to a distance learning situation. The OP mentions kids moving at different paces -- but that happens in person too -- they just don't notice/see it. I think one issue is the idea that you can take a few pieces of quality "content", thread through them and have a meaningful course. Mastery is a great thing to talk about -- what is it exactly? For most, it's doing well enough to get a certain mark on a test. But I'd argue what you want is to ensure that learners use concepts they have mastered in a demonstrative way -- but again, this is harder and will cost more. It would be great if education was concerned about deeper understanding, but right now the focus seems to be "will this be on the test".