New UI. Of course, this is at the top of the list. Even if you only casually noticed Apple's announcements about the new iPhones, you probably didn't miss the flat UI, with its bright, illuminated-from-within background. The good news: It looks visually striking, but you won't have to relearn how to use your phone from scratch. For the most part, you should be able to make sense of iOS 7 without much trouble.
Control Center. Rather than having to dig deep into the Settings app to make simple and common changes to your phone, now there's the easily accessed Control Panel. Go here to change the screen brightness, toggle Airplane Modem set up Wi-Fi and Bluetooth, play audio, and more.
AirDrop. Finally, simple file sharing comes to iOS. Using the AirDrop service, you can quickly share photos, videos, contacts, and all sorts of other files with nearby iPhones. AirDrop works with Wi-Fi and Bluetooth, setting up a sort of peer-to-peer network around you.
Better multitasking. The iPhone's multi-tasking has always been fraught with problems, but now you should see much better performance. Specifically, all apps should be able to multi-task properly, and -- best of all -- apps update in the background, so when a notification appears over an app's icon, the app itself already has that data. No more opening Facebook and then waiting two minutes for the app to refresh with the data it already notified you about.
Safari. The phone's Safari browser has gotten a massive makeover in iOS 7. By smartly hiding some UI elements, Apple has made more real estate available to display Web pages -- essentially delivering a minimal browser experience, like Chrome and the latest versions of Internet Explorer. It also has improved search and better tabbed browsing.
Camera. The camera itself can't be upgraded via software, of course, but the camera app is dramatically improved -- great, because Apple has the least powerful camera app of any smartphone on the market. Now you've got Instagram-like filters, better camera controls, and -- secret until today -- the cool new burst mode announced for the new iPhone 5s and 5c.
Photos. The Photos app is downright impressive. The new Years, Collections, and Moments are ways to smartly group your photos in a way that's easier to browse. And iCloud Photo Sharing lets you share your photo stream with other users.
Siri. Finally, I'd be remiss not to mention Siri's enhancements. "She" might no longer be the right way to refer to the voice recognition service, since it comes with both male and female voices. Siri has more services built in -- including Wikipedia and Twitter -- and can understand a broader range of commands.