I’m an adult learner who uses the iOS app on an iPad. I also have an Android smartphone on which I have installed the Android Khan Academy app, but I don’t regularly use it. I want to address some of your points.

1. Both the Android app on my phone and the iOS app on my iPad have the practices/exercises. I don’t know about the iOS app on iPhones and the Android app on tablets, though. Also, the unit quizzes and tests were recently added to both apps. The iOS app on my iPad also has Missions for those courses for which Missions exist, but the Android app does not. Again, I don’t know about Missions availability for the iOS app on iPhones and the Android app on tablets.

7. Even though I am an adult learner, I am lacking a lot of scientific knowledge and would welcome more beginning science courses (middle and elementary school level).

9. The problem with adding music, even a choice of music, is that we are a worldwide community with different tastes, especially when it comes to grade level. There would also be a problem with licensing existing music or creating original content for which Khan Academy would own the license. I do provide my own music sometimes, but I get to choose exactly what I want.

1. Practice exercises are already available.

2. To be able to do exercises offline they'll have to put lots of data offline which will increase the app size immensely so it's not possible. I think it'll not effective as well.

3 n 5. i) For kids they've 21 of Duck Duck Moose’s apps free: https://www.khanacademy.org/about/blog/post/149459780736/khan-academy-adds-apps-for-young-children
ii) Something big is coming for kids:
https://www.khanacademy.org/kids

9. Totally no for Music because you can play your own music easily in background.

Hi, this was a while ago and the mobile app has been improved, weirdly this post never got a response from the developers, which is bad news since these are valid points. The person who wrote this might not be using KA anymore because of this, however the goal would be to keep everyone and get more people on board. I myself missed years because of the bad mobile experience, and because I don't have 3-4 hours to learn anymore at my PC. But now I gave it another try and here are my results, and this is only for the android mobile, math section:

1. you can do practices but the pages are overflowing, the inputs are misbehaving, the problems (pre-algebra) are unclear, misleading and you often have to look for the input field, and even then it isn't always clear what needs to go into the field. The solution might be wrong because there is an information at the top of the page you didn't see because you didn't scroll up.

2. Agree, and I'd like to add, without pen, paper and calculator, especially on lower math levels. I have 30 minutes every evening to practice, but I don't sit at my table, just have my phone with me. Anyways I need to learn to count in my head at least up to 6-7th grade. Similarly don't give me complicated divisions and scientific notation multiplications unless you give me the space to lead the solution down step by step, because in any exam, the good solution is only half the point, the teacher will ask how did you get to that result.

3. Disagree, make it simple and easy to use. Kids also want wins, and bad but nice ui is worse than ugly but workin ui.

4. As in Duolingo, I'd recommend to build up 365 days of learning of up to 10-15 minutes tasks, and I will decide if I take on the next one, or do only 1 a day. But at least I have progress every day and not stopping for a year. The worst experience one could have is to solve a mastery and miss 1 problem and end up with 99%. 

9. no music please :)

Please set up a mark on the level I am at, because I have no clue. I am mostly on 90-100% in lower levels, but nothing tells me I can move on to the next level. Every time I open up KA, I have to check, where is my level les than 90%, so approximately that's where I can continue from. I don't say you don't lose your knowledge without practice, but in KA you can always go back to repeat a topic because the links are in the problems. So that's great. But let me finish grade 1 and 2 without performing 100% please, it ruins my self esteem and gives the sense of haven't learned anything.Thank you.