Reading this sent a feeling of disgust down my gut. This is exactly the stuff HackerNews hates on Facebook and the like for doing and exactly what's killing my joy of technology / any love for the industry.
Not that you can't learn a language with it, if you are persistent, but Duolingo is a Game first before being a learning tool. It employs all the mobile gaming tricks that get you addicted.
I closed that one after the first few screens as even the first pattern was absolutely disgusting. I don't like when companies behave like needy little children and spam you "play with me" all the time. This whole engagement-based/time-wasting-based business model needs to die.
I don't know, it's one way to do things. You're signing up for an experience like this. Hopefully you're aware of that. If you're not, and you find you dislike the experience, you can stop without any significant loss.
It would be interesting to see multiple user experiences that you can choose between. If you find gamification motivating, you can choose that. If you just want a list of lessons and exercises with no artificial progressions, you can choose that as well. That would be an interesting next step for UI design to take, seeing as many of our objections to an offering are UI based. If companies can capitalize on that fact, they can cater to a larger user base, and alienate fewer people.