Based on the description on their site, the controller includes a built-in battery: "8.39 Wh Li-ion battery, 35+ hours of gameplay... "
That was disappointing for me. Specially condidering the Steam Frame's controllers make use of AA batteries: "One replaceable AA battery per controller, 40hr battery life"
AA Batteries might not be as convenient to use, but being able to replace them is a great advantage. All my Xbox360 controllers still work fine, but none of my PS3' Dualshock 3s.
The official docking station could be used to recharge (rechargables) AA batteries so the functionality could remain the same.
My favorite controller is actually just a cheap 8bitDo 2c hall effect wired controller. Paid $20 but it works reliably and has a third shoulder button.
Wired connection is great for desktop, but the Steam controller is being advertised for use with the GabeCube, and nobody wants a 10 meter cable across the living room where the dog trips on it and yoinks your new $700 console from its desk