Mobile app development has to be highly user centric. The outcome of the process must delight the user! Sounds easy? One of the critical success factors is the 60 second delight! The commitment made by the user to download and install the app means they expect it to be great!
For a user to open your app tomorrow and the next day they need to be given a 60 second delight! Mobile users graze on their apps all day. Mobile consumption is rarely a feast in one app, instead its tiny bite size chunks all day long. Developing an app that has daily user, needs to keep giving the user a fix!
Answer this question at the start of your development process, "What is the 60 second delight?". Keep asking it throughout the process! This will give you focus! Try to answer this honestly and if needed start the design process again if the answer is no. If your app fails to delight at surface level no one will persist to find the wonderful product lurking in the depths.
This question has additional context, what's the 60 second delight for the first time the user runs the app compared to the second or tenth time they run the app?
The 60 seconds delight has to be adaptive to the context of the user. The user experience journey must help the user learn your app features in a highly adaptive and enjoyable manner!
Frequently look and feel are normally considered at the end of app design. While I agree it would be a mistake to invest in high end graphics at the early stages, it is critical to consider emotion and ergonomics.
We touch apps- so they must be ergonomic! The feedback to a tap, touch, swipe etc is critical to get right! Not only the HCI focused positive response feedback, but what would "feel" great! The type of detail needed is quite low level such as how will that animate when it's slid left, will that be the same when slid right?
Finally what do many of the most awe inspiring and beautiful things have in common? They are often incredibly natural! Ask at each stage is this natural, it helps to compare elements of your design to real world physical prompts.