I cannot understand the lack of Kanji in the lessons.
I'm using Duolingo to improve my limited knowledge in Japanese, and some words I have learned it with it Kanji, so when I'm taking a test in Duolingo I cannot understand its Hiragana transcription, because Japanese has a lot of homophonic words.
I had the same issue as you. While they cannot immediately assume that all learners will have kanji knowledge, it would be helpful if there was a 'full kanjification' option. Not sure how feasible that would be, but it would simplify and improve the process for more advanced students who want to review and improve their reading comprehension.
As that level of personalization does not seem to be in Duolingo's model I would not hold my breath. I would rather see an expansion of the course or a separate course focusing on intermediate Japanese and relabeling the current one as a 'Japanese I', or something similar.
Understandable. Reading one of the initial presentations of the course, might possibly give a better idea of what the initial goals of the creators were: http://making.duolingo.com/how-we-invented-a-new-way-to-teach-one-of-the-most-difficult-languages-to-learn.
Warning the course only covers about 88-98 basic Kanji.
As supplements, there are:
1) the LingoDeer app (available for Android)
2) Memrise app and website
3) Minato free courses minato-jf.jp