I'm also *very* interested to see how people will react to the design decisions we made.

Just commenting on your suggestions, it would be difficult for us to switch to a level-based interface for a very simple reason: we don't actually have difficulty levels. This is disguised to avoid scaring newcomers, but it is true. What you are seeing on the left-hand column of the lesson page is the name of the user publishing the lesson.

If you grok the implications of this, I think you'll understand why we want to avoid hard-coding specific users into our navigation interface. We are working on a platform that is structurally indifferent to who is actually publishing content. You can subscribe to my stuff (visit my profile page) and I can subscribe to yours too.

The really challenging usability issues we kept running into during development all involved reconciling this "extra dimension" of potential UGC with what we wanted to be an uber-simple interface. We settled on the design philosophy of "hiding things in plain sight". It's difficult for users to stumble into UGC territory without meaning to. But once they are there they should be comfortable with the interface since nothing has changed.

It took a leap of faith to throw out hardcoding explicit difficulty levels when designing Popup Chinese (they're so pervasive in learning systems that people set them up reflexively!), but I'm increasingly convinced that the notion of "progressive levels" is a seductive but problematic lure. I think it seems intuitively desirable to us because it combines two things:

• Group Consistency: asserting the similarity of in-group objects (if you are at this level, all of these are right for you!)
• Progressive Structure: an implied hierarchy, and the comforting illusion of lessons as sequential units that will bring us to fluency if only we finish listening to them all....

I think there are better ways to handle both needs. Perhaps this comment is getting too long though. Would be really interested to know what you think of the navigation bearing this in mind.