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.
Batteries and lack of audio are my only gripes with the steam controller. I still think it's better than every other controller, but I wish it had those. At least though, valve said it will be easy to disassemble with a screwdriver, so we will have a way to replace the battery with an after market one when it dies.
I'm not aware of any controller that can charge NiMH batteries tho. I think the hardware for that would weight as much as the entire controller. Also NiMH batteries don't do well with continuous charging via dock, unlike lithium batteries.
Audio? It's not a Wii remote lol
Edit: Just realized you probably meant headphone jack on the controller, but that would likely both murder the quality and have worse latency (Bluetooth is not great for PC gamer audio).
They probably mean having a headphone/microphone jack. Thinking about it now, I don't think I have ever actually used the jack on the few controllers I have owned that had one, but I can definitely understand why some would find it very useful.
I briefly had an xbone living in a shared apartment and it came in handy
You have probably never played on a computer without Bluetooth, then (or headphones without Bluetooth). Using headphones connected directly to the pc is a pain in the ass. Using the controller's jack is much better.