Why not add grammar questions?
Duolingo is a great language-learning resource just as it is. But I was just thinking that it might be good to add a few grammar questions. Sometimes it's difficult to pick up grammatical rules simply from examples. I wouldn't want to see a huge emphasis on grammar, but a few questions regarding grammar sprinkled in here and there could be beneficial.
Hope I'm not repeating a point made by someone else in another post.
Also, I've posted this in the "French from English" section, but it really applies to all the different languages taught in Duolingo.