Duolingo Japapese: You're not there yet, but I believe in you!
I know its by no means done, but as it stands, Duolingo Japanese is not even a decent way to actually learn Japanese. I feel wholeheartedly that it is FAR too short, and that it doesn't teach you much more than connecting sounds to Kana. As of yet, it only "prepares" you for the lowly JLPT N5, and even then it really doesn't, because you can go through 30 lesssons in a day, and feel on top of the moon, and then the next hour, you know nothing of what you learned. It not only gives you the illusion that you're becoming good at Japanese quickly, even though you cant really produce or understand much of anything, it also hinders you from really progressing by coming to rely on repetitive, simple lessons, where all you have to do is remember which hiragana makes which sound. I personally use Memrise, among other sources, like Obenkyo and Tae Kim's guide to help me learn. There's this duolingo-esque app called HelloChinese that I use for learning, as you might guess, Chinese. It is like Duolingo in its general structure, but FAR superior and feature packed. It explains grammar, teaches proper pronunciation, listening skills, how to both write AND understand Hanzi and pinyin as well. Its not a direct comparison, since Im talking about two different languages, but I imagine a "HelloJapanese" app would be perfect for learning Japanese, and I think Duolingo could take a look at Hello Chinese to help them understand how to format a more completed Japanese learning course, and make a Chinese one for that matter! I know its hard to use the duolingo way to make a course to teach a wildly different East Asian language, as it originated teaching romance languages, but seeing an app like HelloChinese, which is both very similar to duolingo, and teaches Chinese quite comprehensively, I believe, know even, that it IS possible!
For Chinese speakers: I am currently augmenting my Duolingo and textbook Japanese studies with an CH-JA app I discovered called 最最日语, which appears to be a partial ripoff of Duolingo, but with many additional useful features. There are several different courses within, such as Kana, Basic-High Intermediate Japanese, N5-N2 preparation etc. Each course has a unit section similar to Duolingo, full PPT and audio lectures accompanying each unit, an "assignment" section which gives you daily practices and reviews including pronunciation practice, and a review section similar to Strengthen Skills but where you can pick which vocabulary you want to review. It's not completely free; Lectures are only available in part unless you buy the course, same with tutor feedback on the pronunciation practices. All in all though, you get the content down much more thoroughly, especially in combination with other learning materials.
Be forewarned, the course is entirely in Chinese with only occasional examples from English, so will not be helpful to anyone who does not speak Chinese.