iTunes 10 is getting a major backwards haul from users who weren't thrilled with the new look. Some users may dislike, or simply have trouble seeing the new icons on the sidebar, because of their lower contrast (a monochrome grey/black on white).
There is a fix that inverts those colors, making them a white against a darker background. We aren't fond of the look, but perhaps it will help some users see the icons better.
1) Quit iTunes.
2) Go to the Terminal: Applications>Utilities>Terminal and type in (or paste) the following command line:
defaults write com.apple.iTunes high-contrast-mode-enable -bool TRUE
3) Hit return to run the command.
4) Relaunch iTunes, and you should now have high-contrast icons on your sidebar.