To a large extent, the skills required to develop an app have become commoditized over the past few years. Also available, are a large number of templates to kickstart app development (specially games). That being said there are some key issues which differentiate successful apps:

User Acquisition:
- Organic
- Paid (at a price that gives best ROI).

Sustained Development:
- Ability of the developer to consistently make changes based on user feedback. I think that app development really starts after the first version is released to the market.

Customer Support:
- How well can the developer interact with customer and build a community.

These are my immediate thoughts.