It's a pain in the ass to change a lot of colors and fonts of various languages. I like to have a consistent look when editing my sources, being them java, cpp, c sources or whatever. So I think that a dialog that permitted to edit all the languages at once could be useful.
Other feature I miss is the ability to change the color of the tabs and spaces markers. I usually use a light gray instead of black (in SciTE), that way it's easer on the eyes.
Keep up with the good work, notepad++ is wonderful!