The gamification aspects of Duolingo are a nice touch, but even if it didn't have this it would probably still be popular. I don't think there has to be some sort of "one weird trick" here. It's not perfect, but Duolingo 1) is free, 2) has no ads, 3) has both a website and apps, 4) is interactive with audio and voice recognition. There really isn't any competition to this. Lots of people want to learn languages; is i…
The only other (software) company I can think of in the language space with a recognized brand is Rosetta-Stone and it has traditionally been very expensive. Currently retails for a little less than 400€ (450$) for one language course on Amazon. So I think the user-value from a price-perspective is gigantic if you want to get into a language (not sure how deep Duolingo dives into the language though).
The website is very user-friendly, and the app has been designed specifically for mobile screens, so a lot less typing, and a lot more swiping.
It feels like a paid app, and I can't think of anything that comes close in value. If I were the CEO of RS I'd lose a lot of sleep over duolingo...