As simple as it can be, but no simpler. The application should be immediately useful, but not intimidating, to the user, even if they haven't used it before. Advanced features may not be immediately apparent, but should be easily discoverable when needed. Animations and user interface elements should always have a justifiable purpose.

The application should be as usable to as many different types of users as possible, but we don't want too many ways to accomplish the same thing. Too many options lead to confusion and can be intimidating. To this end, we will only add features and preference options judiciously and after thorough consideration.

Sluggish applications are unpleasant to use. This isn't something you always notice when using a sluggish app, but is something you notice when using a snappy one. Using a snappy application is like removing a weight you didn't know was there. That is why we refer to it as "light feeling".

Applications that move elements without showing the movement don't feel like they made with real parts. We should avoid teleporting UI as much as possible by animating user interface element movement.

Regardless of if it is on a Mac, iPad, or iPhone, the app should match that platform's conventions. That means following Apple's HIG and creating as few custom controls as possible.

We shouldn't discriminate against those with different needs. We don't consider supporting accessibility a burden. By sticking to stock controls as much as possible and putting in a small amount of work, it is very achievable.