A couple of ideas
I love Duolingo, it's the most efficient way to learn a language online in my opinion. It just needs a few bits and bobs to make it really great. I reckon it would be really helpful if there were word bubbles that would pop up here and there, from time to time with tips, typical phrases, exceptions etc. Say whenever you make a mistake or at the end of a lesson. Another thing is... I think you need to work on translations, some are ridiculously meaningless, stuff you would never ever use in a real life. More accurate sentences would be great. And the last thing: would it be possible to replace the automatic voice with someone real? Right now you cannot get anything from the intonation of that voice, because it reads every sentence in the same way, which is not only unhelpful, but also annoying. Anyway, you're doing a great job, so keep up and let us know what you think of that! Cheers!
Generally, I love learning with Duolingo but the quality of the voice is a continual annoyance. Whenever I can I go to an online dictionary that has a spoken component to find out how the words should be said but this is no use when the exercise is one where you have to type from dictation. I have to say I do resent losing hearts that way.
They have a small section under the lesson folder icons (after you click on a lesson on the main tree) that gives a description kind of what you're talking about I think, though more like grammatical rules and occasionally tips and exceptions. As for the mistakes, I think it helps me to click on the comments after each sentence to see what other users are saying. These things might help if you haven't seen them yet though I know they're not exactly what you're talking about... but I agree the voice could be better. I understand from other language learners (non-duolingo/real life) say that the dialect is different in different regions (they told me that the way I was pronouncing "Ich" in German was incorrect and less used?? Which is right?? D:)