Every useable and efficient application UX needs 3 things :

My inspiration comes from an original Tweet by @communicating:

"To create a useable, efficient mobile application UX you really need to minimize 3 things: 1-Keystrokes, 2-Screen Changes, 3-Hidden Data."

I agree with @communicating, but I will open the concept => every useable and efficient application UX needs to minimize 3 things:

1. Keystrokes,
2. Screen Changes,
3. Hidden Data.

Not only because the trend is now towards "mobile everywhere" but also because the more general trend, you guessed, is : "Keep it easy simple !"