Whenever a major new version of OS X is released, many of Apple's notable apps get a bit of a facelift, too. iPhoto, iMovie, & GarageBand received updates to take advantage of Lion's full-screen feature. iWork received full-screen, auto-save, versions, and resume support.
And then there's iTunes. Normally, iTunes just gets worse with each update. I have, for years, blamed this on iTunes being a dinosaur running on 32-bit Carbon, relying on crufty frameworks a decade old. It was just plain bogged down.
Key word: was.
Just like when the Finder was rewritten with Snow Leopard bearing the tagline "all-new, just-the-same", iTunes has finally been rewritten in Cocoa and is fully 64-bit with version 10.4. So, to the casual observer it may just look like iTunes got the full-screen Lion treatment, but a keen person will realize that iTunes is actually fast again. Everything I have done in the new iTunes 10.4 has been quick and the beach ball of doom hasn't reared its ugly head.
I have no idea how things are with iTunes on the Windows side of things, but Mac users can finally begin to enjoy iTunes again.