>In any case though, basically everyone knows how to use the computer and the web now. The general UI elements and their rules - links, buttons, sliders, etc, are all known to just about everyone. At this point since we've trained everyone, we get to drop some of the frills around the UI without suffering a substantial drop in usability.

Chiming in as well with, this isn't exactly true. Just the other day in a DMV I watched a woman really struggle with a touch screen because she didn't realize that two of the "flat" buttons, were options she could click, she thought they were just information.

I'm a big fan of the flat look, but I do see how it can be confusing for some people.