This comment is based on studies of healthcare apps, and some recent conversations I've had, but I'm guessing it applies more widely:
- Consider what the 'added value' of the app is intended to be. 'Because it's the 21st century' and 'Because everyone uses apps' are not added value. What are the benefits to the user of doing something with an app rather than either doing it some other way or not doing it at all? Make sure there is clear added value.
- Consider how people will fit app use into their lives. Is it meant to be used when a particular trigger event happens (e.g., the user is planning to go for a run, or isn't feeling well today), or regularly (after every meal, first thing every morning, or whatever)? How will people remember to use the app when intended? Make sure the app fits people's lives and that any reminders or messages it delivers are timely.
- What will people's motivations for using the app be? Are there immediate intrinsic rewards or longer term benefits? Will these be apparent to users? Does there need to be an extrinsic reward, such as competing against others or gaining 'points' of some kind, or might this be counter-productive? Are there de-motivators (such as poor usability)? Make sure the app taps in to people's motivations and doesn't put obstacles in the way of people realising the envisaged rewards.