The latest version of iTunes has made several changes to the UI (again), most of them not for the better as can be read about here. Still, at least one of the annoyances, the use of all capitals for section headings in the source list, can be easily corrected. The section headings are stored as localizable strings in a resource file; so first shut down iTunes, then just open the /Applications/iTunes.app/Contents/Resources/English.lproj/Localizable.strings in a text editor (e.g. TextMate) and search for the following lines:

1
2
3
4

"135.011" = "LIBRARY";
"135.012" = "DEVICES";
"135.013" = "STORE";
"135.014" = "PLAYLISTS";

these need changing to:

1
2
3
4

"135.011" = "Library";
"135.012" = "Devices";
"135.013" = "Store";
"135.014" = "Playlists";

or anything else that you like. Now when you restart iTunes it should all look better.

Update: You also want to change this line: "135.006" = "SHARED"; to this: "135.006" = "Shared";