Lessons that help users understand the language
I've been using Duolingo for about six months now and I'm really enjoying it. It's fun, challenging, frustrating and actually effective. I can read French pretty well now, but I feel like there is something missing. Learning a language in school is effective because you have a teacher who is there helping you and teaching you about how language is used. For example, my high school Spanish teacher taught us rules and irregularities in Spanish that we otherwise wouldn't have known, like how conjugations cannot follow prepositions like "de" and "con." Duolingo does not have that. You are basically thrown into the subject you're learning about and you have really no idea of how the language is actually constructed. I feel like if Duolingo we actually to be used in classrooms or in lieu of formal education, students would have a tough time constructing sentences on their own.
For example, from using French Duolingo, I am not exactly sure why negations require "ne" and "pas." I did not know what "en" or "y" meant until I tried to use them and I got it wrong. The trial-and-error aspect helps, but it is not a decent substitute for an actual lesson.
Now an answer for this is to do what I have been doing; use multiple sites like about.com, Memrise, lang-8 and Cliffnotes, but that diminishes the usefulness of Duolingo. Duolingo is excellent at drilling and getting accustomed to the language, but I am not sure it really teaches, and if it does, it does not teach as well as it should.