There are so many challenges in designing an app that introduces completely new concepts to people.

I'd say the biggest challenge is getting folks to understand that banking with bitcoin is just fundamentally different than traditional banking - while still trying to make the app as familiar as possible.

For folks used to using traditional banks, they're used to concepts like waiting 3-5 business days to see your deposit in the bank, having to WIRE money if they want to make international payments, waiting a long time for anything to be done, paying a lot to move their money around. The challenge is that we're trying to explain to people that they no longer have to deal with any of that anymore... while also demonstrating that the app is safe to use and that they WON'T lose their money.