Good Examples of iOS Code Quality?

I recently interviewed at company X, which had one of those take home assignments, and in the end I got rejected because my code wasn't up to their standards. That got me curious, I've been a developer for ~6 years, (last years I've been managing though, not doing much coding), and although I work in a big company, I've been in the same job for a while now. Not too much of a sample size, and iOS Dev Weekly et al have interesting articles, but I think they focus on new stuff more than things like, how does a good mature iOS app/library look like.

What do you look for in a `good` iOS app/library?