Though it isn't a life-changing feature, the fact that you now can select a wallpaper for your iPhone's lock and home screens is pretty exciting. Installing [[xref:http://www.pcworld.com/article/199380/apple_iphone_ios_4_arrives_at_1_pm_et_10_reasons_to_upgrade.html|iOS 4]] on an iPhone 3GS takes under 10 minutes, and you are also prompted to do a quick carrier-settings update. The device then restarts, restores all your data (music, photos, contacts, e-mail accounts, and so on), and presents you with your new home screen, which can now feature a background image.
You can choose two different images for your lock and home screens, or use the same one throughout. Note that the Calculator icon has been refreshed as well (though the app itself remains the same).