How Duolingo models language learning

Pretty cool to see them innovating on the spaced repetition front. I'm around halfway through my second course (finished esperanto, now french), and I'm very grateful for such a high quality, free course. That said I recently started supplementing with the Pimsleur audio courses because the main weakness of duolingo is a severe lack of listening / speaking comprehension.

I just wish that they would update their Windows app. :(