When you get an answer wrong, should Duolingo tell you the damage to your reputation that would occur if you were to make that mistake when speaking to a native speaker?
Depending on the mistake, they might think you are uneducated or they might get really offended, etc.
Duolingo could even keep track of your overall reputation. So when looking at someone's profile, you could see this (e.g., "often rude", "appears uneducated", etc.).
One of the benefits of Duolingo is the ability to study at your own rate in your own home. An overall reputation tracker initially would be low as a person is prone to mistakes early in their lessons. There is feedback on what was typed wrong, and corrections to that. I have noticed this program focuses on positive encouragement and creates a pleasant learning environment. Perhaps people would be put off by a program not only telling them they were wrong but also criticizing their intelligence. I suppose as an optional function for the higher level phrases it may be alright, but take notice that when you translate a page other people can rate and comment on it. I may be completely wrong in these opinions though, and my response is based on anecdotal evidence, but I can see where some people would be interested in that option. I would have to decline.