You'll be able to call people using your WiFi connection on the iPhone 6, so when your WiFi is working better than your reception is (all the time?) you'll have an alternative. And Apple's WiFi received a speedy upgrade: 802.11ac WiFi in the iPhone 6, or three times faster than the previous generation. But that's nothing compared to the handoffs.
Probably the best part of Apple's new WiFi calling system is that if you are calling from WiFi in your house and then just leave, it'll automatically switch over to the cell tower. No interruptions. Apple is working with T-Mobile to offer this superpower in the U.S., so it's limited to just the Uncarrier for now. No word yet on where it'll be available globally, or for beleaguered Verizon users.