This one is huge, because in one shot it adds a number of features that we've wanted to see in iOS for years, though the implementation is more like Windows Phone than Android. System-wide extensions (Apple uses the term "extensibility", but we'll stick with the more common terminology) in general allow apps to communicate with each other. Apple is doing this its own way by creating "live app sandboxes" to allow this communication, but the end result for users will be great features like being able to share items to any app, and not just those that Apple allows. This means sharing websites directly to Pinterest, or creating a document in Pages that saves directly to Dropbox. It also means that apps can share functionality. For example, you could use VSCO Cam filters directly inside Apple's Photos app, similar to WP's Lens feature.
Extensions also means that iOS will now have widgets, though probably not in the sense that you would normally imagine. iOS 8 widgets will only live in the Today screen of the Notification Center, and not on the home screen. So, you will be able to add third-party widgets to add info to that Today screen, including those demoed like ESPN SportsCenter, and a live widget from eBay, where you can bid on items directly in the widget. Extensions won't stop there, though, because Safari in iOS 8 will allow for extensions as well. This could mean extensions like any of those you'd find in desktop versions of Chrome or Firefox, but the one that Apple showed off during the demo was an in-line translation extension from Bing.
And, to top it all off, extensions means that iOS 8 will support third-party keyboards! As we have already covered, the stock keyboard for iOS 8 has gotten some nice improvements, but you'll also be able to ditch that keyboard completely in favor of an option like Swype. By default, the keyboards are not allowed Internet connectivity unless you allow it.
That is a huge amount of functionality added in one move by Apple. It may not be anything new compared to the competition, but iOS users should be very excited for the iOS 8 update in the fall.
image source: The Verge