A critical point on usability is that not everyone has the same needs or expectations.
Needs: Someone on an iPhone is not going to be able to navigate a Flash menu, because Flash doesn’t work on Apple mobile devices. So you can do cool things with Flash driven menus, but then you have to provide text links as an alternate method of navigation.
Expectations: Most people expect after they enter their credit card information, that the next time they click a button, their card will be charged. Also that it may take a few seconds for processing during which nothing is going to be happening on their screen.
But not everyone knows these things, so you need to tell them.
Now there is no way to accomodate everyone. If you have a small business, chances are you aren’t going to provide a version of your website in Urdu for the few visitors who only speak that language.
So you have to figure out how far you are willing to go, can afford to go, or need to go.
But a starting point is CONCEIVING of your different types of visitors, their needs and expectations. That means knowing who your potential buyers are, how they think and what they know.