Key ingredients of a great user interface

What are the key ingredients of a great user-interface? I was invited to contribute to a 'Computer Arts Projects' opinion piece. Here's my tence pence worth.

"A Great interface complements and supports the aims of the project, while considering it's audience. 'Key ingredients' can vary a lot, however intuitiveness, clarity and aesthetics should always be considered. Standardising is sensible for functionality but experiential sites can and should explore more immersive ideas. It's worth remembering that we as people like to be entertained and are often persuaded by our emotions".