What key things form the best Android app UI?

To those of you who've built many apps, what are the things that are critical for a good UI? I'm thinking things like:

• Action Bar


• Well-designed icons


• Use threads to keep the app responsive


• A selector to highlight when focused/pressed, etc.


• Pay attention to layout & consistent alignment etc...

But what else?

Answer

At a high level I think these are most important, and they go for any human interface, not just Android UIs:

1. 
   

   Clarity — ensuring your UI is immediately clear to most if not all your users; minimizing user confusion.

   
   
   
   
   • Visually group logically-linked interface elements.
   
   
   • Use proper affordances—buttons should look like buttons.
   
   
   • Use subtle animations to aid the user's mental model for interface transformations (e.g. navigation).
   
   
   
   • Show a one-time tutorial if necessary; it's worse than having a perfectly understandable interface, but better than throwing users into a potentially confusing interface with nowhere to turn for help.
   
   
   
   


2. 
   

   Consistency — leveraging existing patterns from the system where applicable, as well as using consistent patterns within the app.

   
   
   
   
   • Use common Android UI patterns like the action bar, bottom button bar, contextual action rails, gesture-based paging, etc.
   
   
   • Align with common system behaviors like the BACK and SEARCH buttons, app widgets, and notifications.
   
   
   • Maximize internal consistency using themes and styles.
   
   
   
   
   


3. 
   

   Usability — maximizing ease of use.

   
   
   
   
   • Provide visual or otherwise haptic feedback for UI elements.
   
   
   • Add directional navigation support and text hints (contentDescription) to improve accessibility.
   
   
   
   


4. 
   

   And of course, Aesthetic — making sure it looks damn good; strong emotional ties to products often come from beautiful interfaces.

   
   
   
   
   
   • Support all screens by providing assets at each target density, using nine-patches, and providing tailored layouts for different physical screen sizes.
   
   
   • Visually customize either all interface elements or none at all for given screens—if you customize only a portion, you'll end up making poor assumptions (devices have varied OS themes).
   
   
   • Hire a designer.
   
   
   
   

Responsiveness and resource stinginess are crucially important as well, but those are more influenced by implementation than design.

See my slide deck on this topic for some more tips.