Aren't most affordances just arbitrary rules that most people can be expected to already know? I'm not sure that an adult introduced to computers for the first time in their live would figure out convex-looking buttons much faster.
Affordances aren't arbitrary. The idea behind affordances is that the tool fits the user, not the other way around. If something is meant to be gripped in a person's hand, then it should be the right size and shape for a hand to grip it. Conversely, if something is the right size and shape to grip in a hand, that intuitively suggests that it is for gripping.
True, but affordances on screen are a different story. There's nothing to grip there. There are just pixels on a flat surface, that may or may not simulate a 3D effect. And you operate them using a mouse, which itself is a layer of indirection. So I don't buy the concept that affordances in software are not arbitrary - they don't refer to somewhat fundamental concepts for human (like grabbing), they refer to the last 100 or so years, when physical buttons became a common element of the environment one lives in.
I see learning UI in computers as a process that begins with a new user having basic UI concepts explained to him, and then just following the changing trends, where a new trend usually makes "affordances" refer to the previous trend.
