A big issue for travelers is that carriers use different communication bands. Most Canadian cell phones support the "standard" bands (850,900,1900,2100 MHz). Newer Canadian providers and some American providers use other bands such as AWS bands (1700, 2100 MHz).
What does this mean to you?
It means that you will be able to use the fastest possible connection on more networks around the world. Previously (with an iPhone 4s), my connection sometimes dropped to 2G (or edge) networks which meant painfully slow connectivity when travelling to the US. With the new iPhone 5s, you will be on 4G more often and the connection will be much more reliable.