While Apple has announced that 200 new features are coming to iOS 5 in the Fall it hasn't said much beyond the 16 or so (Notification Center, iMessage, WiFi Sync, et. al.) that are listed on its iOS 5 feature page.
Here are a couple of my favorite new iOS 5 features that have largely flown under the radar:
Text macros - This new feature allows you to automatically type frequently-used information by typing a short abbreviation. It's configured in Settings > General > Keyboards > Shortcuts (scroll to bottom) where you set up short strings that are replaced by a longer text strings when typed. Text macros could get really interesting if Apple releases support for syncing them via iCloud. Apple could also conceivably release text macros for Mac OS X putting it in competition with Smile Software's essential TextExpander software.
LED Flash for Alerts - Tucked anonymously under Settings > General > Accessibility lurks this little gem that lets you see (and feel) when someone’s calling. It's sure to appeal to the Blackberry crowd that are hooked on its dedicated status LED -- something that the iPhone has previously lacked. Apple's iOS 5 implementation is more of a hack -- simply blinking the camera's LED flash when an email arrives -- as opposed to BB's dedicated multi-color status LED.
Custom Vibrations - Accessibility also allows you to assign unique vibration patterns to people in Contacts. You can even create your own vibration patterns in Settings > Sounds > Vibration.
So that's close to 20, what about the other 180 other features in iOS 5?
While Apple's isn't saying much beyond the few features that it already showcased, TechZoom has compiled a list of 133 iOS 5 features that it has independently identified to date. Here's the first 50:
Notification Center: New Notification System
Improved app notifications at the top
Lock screen notifications. Swipe will open related app.