This is a very loaded question with many different elements that come in to play. I’ve reached out to our Art Director, Mitch Maltese to give you some of his top answers, but there are literally books written about creating a great mobile user experience.

One of the most important things to ensure a positive user experience is to establish consistent styles for interactive and non interactive elements. This is especially important on phones where there is no hover feedback.

When users tap away and nothing happens they will quickly leave, so make sure buttons and links work too.
Properly sizing and spacing interactive elements in an interface is crucial. Look at the tip of your finger and that should give you a good idea of how to size them. Space buttons out so that you don’t accidentally hit an unintended one because they were too close.

Nice readable text is a no brainer. Average users shouldn’t have to zoom just to see what a button says.