I think it would be cool if there were chat rooms for practicing whatever language you are currently learning with other people (Maybe close to the same language level as you?).
After SRS (Spaced repetition) and translation, live exchange with others is naturally the next step in language learning. I'm not sure whether or not providing such an environment would go beyond duolingo's initial mission of "translating the web", but I'm all for it nevertheless. It's 2013 and there is hardly any decent chatroom for language learners out there that isn't either built in Flash (slow, not portable) or flooded with spammers and freaks looking for a relationship (annoying). Having said that, regardless of whether duolingo ever develops a chatroom or not, a basic integration with VoIP software (Skype, Ekiga, Ventrilo) would be a great start, and an IRC channel is a must!