A color palette that has an option for transparency in all applications. The Finder only lets you choose the color in terms of solid colors, but there should also be a transparency option. I would also like to see a color that is the inverse of what is behind it. This would be great for the subtitles of DVD's.
Better disk and network server support. My system slows down noticeably when I am connected to a fileserver or have an optical media in the drive.
How Could OS X Be Improved?