As I see it, there are (at least) three issues with iTunes' new aesthetic. Fortunately, on the Mac, each problem has a simple solution.
iTunes 10 exists in a world of bland and non-standard interfaces.
Problem #1: The new icon
With so many music purchases coming in the form of digital iTunes store instead of physical CDs, Apple decided it was time to ditch the disc in the iTunes icon. But change is hard, and users don't want to see something new in their dock every day.
Solution #1: The old icon
To restore the old icon, first make a copy of iTunes 9 before upgrading; if you've already upgraded, restore a copy from your Time Machine backup. Select both the iTunes 9 and 10 applications in the Finder and click the File -> Get Info menu command (or press Cmd-I on the keyboard). Click the icon from iTunes 9, click Edit -> Copy (or Cmd-C). Finally, click the icon on iTunes 10 and choose Edit -> Paste (or Cmd-V). The next time you launch iTunes 10, you'll see the decade-old icon.
If that's not the only icon bothering you, then skip this fix and go right to ...
Problem #2: Grayscale interface
iTunes' leftmost column still sports icons for all your different media -- music, movies, playlists, and more -- but in iTunes 10, each icon is the same drab gray color. Open your preferences window and you'll find more blandness.
Maybe Apple was going for a svelte, sleek look. As someone who still prefers the rainbow Apple logo, I'd like to see a bit more color in my app.
Solution #2: A resourceful splash of paint
A young Australian by the handle Shaun_R has posted some resource and icon files that, with a simple drag-and-drop, restore most of the colored icons to iTunes. It even replaces the main program icon with the old one, thus addressing Problem #1 as well. However, unless you make a backup of the original files before applying this patch, there is no "undo" feature, short of reinstalling iTunes 10.
Shaun_R's UI overhaul is a work in progress and does not yet restore 100% of the icons to their colorful state. Given how quickly he's turned out this initial release, I expect the complete patch to be available before long.
Problem #3: The title bar
Apple programs share a common interface that make them easy to navigate. This standard includes the name of the window at the top, and and three horizontal buttons in the window's upper-left for closing, minimizing, and maximizing.
Apple disregarded this style with iTunes 10. The name "iTunes" is gone from the window, and the traffic lights are oriented vertically, making it a misfit among Macintosh programs in both look and functionality.
Solution #3: Restoring standard window features
Just drop into the Terminal and paste this command:
defaults write com.apple.iTunes full-window -1
Look at this blog post. Now back to iTunes. Your icons are now diamonds!
Each of these fixes should be applied while iTunes 10 isn't running. Now you can have iTunes 10's features with iTunes 9's aesthetic. The best of both worlds right?