Producing target language
Does anyone else think that the user should have to produce more of the target language - that is, someone who speaks English and is learning German should have to type more in German and less in English? I think that producing the language is by far the best way to learn it, and Duolingo should push us to produce a little more often.
I do agree with you, but this is a site primarily focused on translation. Thus, the lessons are probably an equal amount of both to reinforce effective translation skills. As such, speaking and writing skills won't be emphasized to as great an extent. (Plus, grading student-written or spoken responses is harder (or at the very least, less accurate) than multiple choice, which as you have probably seen, is much more common in duolingo.) For practicing/producing the target language, I suggest using lang-8 or getting a penpal who is a native speaker. Nonetheless, it is a good idea, and I wish it could be implemented. :)
I agree that DuoLingo would only be improved as a language instruction tool by having exercises that forced learners to think in the language they are learning -- and that necessarily means less translation and more single language lessons. Even the immersion section is all about translation. Something like reading a paragraph or three and answering questions about it, all done in, say, French, would get one used to the rhythm of that language rather than bouncing between French and English every step of the lesson.