One of the biggest gripes about larger smartphones is how hard they are to use one-handed. Apple's solution to this problem is through a feature they're calling "reachability". The iPhone 6 and iPhone 6 Plus will allow you to double-tap the Home button while in an app in order to bring top navigation controls within your reach. That combined with gesture support that was introduced in iOS 7 is going to make for a one-handed experience that doesn't currently exist.
I've touched on the fact previously that I felt gestures were introduced by Apple long ago in order to prepare folks for larger display phones. If we started getting used to swipe gestures now, they wouldn't be such a learning curve later. It appears that Apple is using this strategy in combination with reachability to bring a better experience to large display phones.
The iPhone 6 will feature a 4.7" screen while the iPhone 6 Plus will feature a 5.5" screen, which is just .2" shy of Samsung's Galaxy Note 3.