There seems to be two product systems which are quite popular and successful at the moment, for mobile apps. 

1. Simple, mono-action apps

This is Instagram and Vine.

Focus around a unique use case, craft a superb user experience, wait for the market.

Instagram: Take a picture, apply a filter, share it to all social networks. 
Vine: Take a 6 second, non-continuous video, share it to all social networks.

2. Kill the middleman

This is Uber, Airbnb, iCracked and Exec.

Find some market which does not need a middleman, as we now have phones with constant Internet connections. Then make connections, and exchanges of goods/services between people super easy, peer-to-peer style. 

Airbnb: peer-to-peer house/room rentals
Exec: peer-to-peer house cleaning

In the case of Uber and iCracked it’s business-to-peer via the app.

So you know what you have to do, if you want to build a startup now. (Or you could build a cheap, efficient solar panel (apparently, you can’t).)