It's easy enough to get going and hard enough to feel like you're doing something but not so much you give up.
The points system, scoreboard, and email prompts are a good way to encourage you without being annoying. The ranked scoreboard really has me wanting to stay there and compete to stay in the top 10 each week.
One thing I miss is the speaking option. Not all languages offer it but some did and that option seems to be missing or at least on my device it is.
Another problem is some languages are hit and miss with pronunciations if there are few native speakers. I know there are computer generated voices but sometimes there is no sample of the words at all (Irish being one example).
Anything that gets people interested in languages is a good thing. Duolingo may not be perfect but it's a popular and easy to use app. We unilingual English speakers need to get to with the times and learn at least one second language, maybe even more.