Once I click customize, I can no longer figure out what some of the existing
buttons (especially ones that are new for Office 2003) are. This makes it
more difficult to place new buttons, or remove default buttons which I won't
end up using.

I prefer to have only a dozen or so buttons, and use menus for less familiar
actions, but don't know what features I'm getting rid of because I can't
identify the buttons. Currently I find myself leaving customize to identify
buttons, and then going back into it to remove buttons, then leaving it again
and coming back in, etc. Tooltips could continue to work in Customize mode,
or some other method could help you identify what an existing toolbar button
is.