Failure to invest in these key app development items upfront could result in a substantial amount of lost time and money down the road.
An Enjoyable, Intuitive and Functional UI
There's a saying among user interface geeks: "A product interface is like a joke. If you need to explain it, it just isn't that good." We all know from experience that this is true, but it's easy to overlook a dysfunctional UI on your own product, especially when you're so familiar with it.
Bring in trusted friends (especially non-techy people) at an early stage to provide honest feedback on your app's usability. Don't push back the way Richard Hendricks does in the HBO comedy Silicon Valley. Instead, take the feedback to heart and commit to fixing any perceived problems. This is particularly important because once the product is launched, it's often too difficult to change the UI on a fundamental level.