I always find it fun to try to spot all the tiny design differences in Apple's user interfaces whenever a new update comes out.
A blog called Design Archive has put together some comparison shots of iOS 4 and iOS 5 highlighting some of the more fun differences. Normally I would quote the site I'm linking to, but this one is in Japanese, and Google Translate doesn't exactly offer grammatically readable sentences either, so here's a few of the fun ones they found:
I love that Apple's designers revisit things like this so often.