This is what I really like about Duolingo, and why I believe, as a company they will do very well:
It's Free - almost all competitors charge for their service. Duolingo have a very cool business model. They charge for translating documents - something which the language learners happily do without even knowing.
The Best Interface in This Space - The design is solid. The lessons look great and children will have no problem using this site.
- Streak - I like any app that incorporates this very powerful motivator. Humans don't like losing stuff. So the longer you build your streak (by logging in and learning everyday) the more you want to keep logging in, in order to retain the streak score. 750Words does the same thing and it's extremely powerful.
I came across Duolingo quite a while ago, but have only just managed to fit it into my schedule and start using it on a daily basis. I'll post an update as to how I'm progressing in a month or two.