Rule #1

I started making up rules a few years ago, just to formalize some of the ideas. And to force people into taking them seriously. Including myself.

Rule #1: a button should look like a button

Okay, so compared to how simple this rule is, I had to start calling it #1 for people to take it really seriously.
But just think about it. If you're designing a new interface for a touchscreen product, your interface is a lot less explorable than a PC. There's no tooltip to begin with. No cursor that could change to a hand or carret. The user explores your interface by looking at a screen. A small one. From a distance. He can either recognize buttons and tell them apart from non-buttons or he'll be frustrated like hell.
What IS a button? Anything that you can click on a screen that performs an action is a button, more or less. So anything that ACTs like a button should LOOK like a button.
It might be the simplest of the rules, but it's very hard to fulfill consistently.
In so many products you end up with exceptions, and they ALWAYS bite you in the ass. Even if you call them 'paradigm shifts'.