> You have to have outside experience of the words to really have them click.

I always thought this was a given though. I don't think it's possible to learn a language in a vacuum. Languages are also people and culture.

> Duolingo lessons generally fail to build up on each other sufficiently.

Agreed. There's plenty of lessons that don't seem to be getting reused later, like they're leaves in the tree of knowledge. Those are the ones I have the most difficulty with when reviewing past lessons.