I think many will agree that iTunes 10 was more of a marketing ploy than anything else... I found that iTunes 10.0 was worse than 9.x, primarily because of the "traffic light" style buttons in the window corner and graystyle icons. That was bearable though, and then the update to 10.0.1 came along made things even worse. No more Genius sidebar, and obnoxious "Ping" buttons every time a song is selected.
So I went on a quest to fix it, and found these commands on various forum threads and blogs:
(If you are unsure how to run these commands, quit iTunes and open Application > Utilities > Terminal. Copy/paste these commands in, hitting <Enter> after each to execute it.)
defaults write com.apple.iTunes hide-ping-dropdown 1
Disables the obnoxious "Ping" drop-down menu I mentioned earlier.
defaults write com.apple.iTunes show-store-link-arrows -bool TRUE
Restores the store arrow links present when a song is selected in 10.0 and earlier.
defaults write com.apple.iTunes disablePingSidebar 1
Disables the ping sidebar. Sadly, I haven't found a way to re-enable the Genius sidebar.
defaults write com.apple.iTunes full-window -boolean YES
Puts iTunes in "full window" mode, removing the traffic light window controls.
Now, the last thing is to restore color in iTunes. To do this, download the iTunes.rsrc file linked to in comment #7 on this thread. Next, right-click on iTunes in the Applications folder and select Show Folder Contents. Inside the Contents/Resources folder, copy the downloaded iTunes.rsrc file and opt to replace existing files when prompted. Restart iTunes and you should have coloured icons!
Note: If you're reading this but you're running iTunes on Windows, a user on Apple Discussions has posted how to do the same on Windows.