I'm surprised nobody is talking about the Apple SIM which personally I feel is one of the most incredible features they've shipped this year: no more SIM cards, just pick the carrier you want in the Settings app and you're signed up. Fed up with them? Switch carrier in an instant. Going abroad? Switch to a local carrier. Just wow. I really hope they bring this to iPhone.
http://www.apple.com/uk/ipad-air-2/wireless/ (near the bottom, search for "Apple SIM")