Apple made a new programming language; a majorly impressive feat. It'll be faster than Objective-C. It's also the future of iOS and OS X development. For the sanity of their developers and longevity of apps, Swift code can exist in C and Objective-C code in the same app.
While this was a consumer product also, Apple is offering a development platform that will allow developers to turn iOS devices into smart home remotes. Developers at dozens of other companies have already begun developing apps like this, so HomeKit will be a major help.
Various Apple devices, like iPhones and iPads, can now communicate between one another. Before, they were cut off from one another. That meant moving files and data between apps on the same device was a pain. Now, extensions have resolved this sandboxing.
Widgets in Notification Center
Widgets are now available in the Notification Center, so developers can have users pin their apps to the notifications, speeding up the opening process. Promoting this pinning will be a major force in iOS 8 development.