Usability and Invisibility

As a web designer, I have a passion for usability and the "user interfaces" for so many things other than just websites. Some things are so easy to use, yet others are so hard. Why is this?

I'm sure I don't have all the answers, or even the right one, but I have an idea. Making things easy to use, or to make them even easier to use, requires a certain amount of effort. It's not easy to make things easy (is that from a song?). So I formulated this preliminary axiom:

The easier something is to use, the more difficult it was to design; the harder something is to use, the easier it was to design.

Of course this is no ground-shaking discovery of great truth. However, I find it interesting in the context of web design and so many other user-centric designs. Successful designs often work best when they are invisible to the user, or transparent to a large degree. Making a good user interface may easily involve putting in a lot of time and energy into something, only to sacrifice it to the user's needs, and mostly disappear in order to succeed. I find it ironic that usability requires so much invisibility.