I would love for Duolingo to focus more on content, and not just on gamification. Right now, the main courses (such as Spanish and German) are great, but many courses are short, lack grammar tips, and have mistakes.
Even the Spanish course has problems: I am currently on unit 175 out of 211, and I have noticed that the lessons gradually have more and more problems. Some phrases are stiff, some are completely unnatural (and people complain in the comments), sometimes there are only a handful of accepted translations and we are forced to guess, and sometimes translations are inconsistent - even though several questions have a similar structure, the accepted answers are different.
Another point is the recent redesign. Personally, I'm okay with it and I have no problem with changing the tree into the path. What I didn't like is that the course was artificially inflated, and now we have to repeat the same things many times. One could argue that repetition is key to learning a language, but it can get tedious when you have to answer the same question multiple times without making any mistakes. Furthermore, the exercises themselves are becoming easier - previously there were more questions where it was necessary to type in the full translation, but now in most exercises we only need to input a single word or rearrange the word cards.
Lastly, many people who seriously participate in the leaderboard ranking don't actually learn the language, they simply farm experience. Their usual approach is to rack up XP bonuses and redo a single exercise dozens and even hundreds of times in a row. It's sad that this behavior is encouraged.
As for myself, I use Duolingo simply as an additional practice tool. I use many other resources for actual language learning. I recently reached Spanish B2 level, and many years ago, I reached German B2 and Japanese N3.