Luna was the code name for the Windows XP "look". The designers did a lot of research (and got off to a lot of false starts, as you might expect) before they came to the design they ultimately settled upon.
During the Luna studies, that people's reaction to Luna was often, "Wow this would be a great UI for X," where X was "my dad" or "my employees" or "my daughter". People didn't look at it as the UI for themselves; rather, they thought it was a great UI for somebody else.
It was sometimes quite amusing to read the feedback. One person would write, "I can see how this UI would work great in a business environment, but it wouldn't work on a home computer." and the very next person would write "I can see how this UI would work great on a home computer, but it wouldn't work in a business environment."
(And interestingly, even though armchair usability experts claim that the "dumbed-down UI" is a hindrance, our studies showed that people were actually more productive with the so-called "dumb" UI. Armchair usability experts also claim that the Luna look is "too silly for serious business purposes", but in reality it tested very well on the "looks professional" scale.)
Aero is the code name for the Longhorn "look". With Aero, the designers have turned an important corner. Now, when they show Aero to people, the reaction is, "Wow, this would be a great UI for me to use."
People want Luna for others, but they want Aero for themselves.
[Raymond is currently on vacation; this message was pre-recorded.]