For some of us, the addition of fully customizable SMS/alert tones in iOS 5 is the biggest improvement in the feature list. (Well, maybe not the biggest, but certainly one of the biggest.) Since alerts can be linked directly to contacts for incoming iMessages or SMS receipts, having a larger library of custom sounds means you can personalize your iPhone to deliver more context in less time -- and without having to look at it to know if that text message is from the spouse or the boss.
As iOS 5 has rolled out, the iTunes store (the iPhone version) has added a new section for Alert Tones -- you can find it by scrolling down in the Tones section, or from Settings --> Sounds by tapping the 'Buy More Tones' button. Once you get over the idea of paying US$0.99 for a tiny snippet of audio, you can shop to your heart's content among the sound effects; the Star Wars lineup is particularly popular at launch.
Of course, it's easy enough to use your own sound library or recorded audio to create your own custom ringtones and alerts with iTunes.