Not getting responses to feedback
More than once I've clicked on "still think you're right?" when Duolingo has marked an answer wrong. I have never got any responses from Duolingo, either to explain why I'm wrong or to acknowledge that I'm right. The first time was weeks ago, so I don't think it's a matter of being patient. Will I ever get a response, or is there no reason to keep trying with this?
I don't think it is realistic to expect a reply. Keep in mind that the function is for us to give feedback to Duolingo, and not for them to give us feedback. Besides, you should already know that you are right if you have used that function, or at least you should be fairly sure.
Consider also how many more users there are than people working on Duolingo... With that in mind it is pretty awesome that they still take the time to reply when they can.
I agree with what nimbus77 is saying in that it becomes very hard to make personal responses as the number of users and thus amount of feedback increases.
One suggestion I would have is being able to go 'behind' a page in a lesson and allow people to raise points about it and have them voted on. So if your answer in a lesson was marked incorrect when you thought it was right you could go 'behind the page' and perhaps see that someone also thought the same answer was correct in which case you could vote them up or if it wasn't there add it yourself. Then the editors of the lesson could go through and review the suggestions (looking at the highest rated first) and tick off any they accept.
In other words, an implementation of bug report functionality. Quite a lot of work to to add it though...
I would expect in the long run that "Still think you're right" feedback would be processed by computers, not humans, to improve their range of acceptable responses. Expecting a personal response is not realistic. Still, a computer generated response indicating something about the level of problems with the correctness would be interesting. Search on "TED talk Luis von Ahn" to get the big picture.