The new MacBook Pros will apparently use USB-C ports for connecting external devices, like external hard drives, displays, and anything else you normally connect to your computer, according to Ars Technica.
That potentially means you'll need an adapter to connect legacy peripherals that don't use USB-C, which could be a pain.
At the same time, USB-C allows for faster data transfer speeds, and you could have all your devices connected to a single USB-C adapter rather than having multiple cables sticking out of your MacBook Pro, which makes it easier to plug and unplug all your peripherals when you need to.