If their controls are supposed to simulate a mouse I imagine it will be very similar to a laptop track pad. On a controller right now you have sticks, if you push up on the stick you'll look up (or down if you play inverted) until you let go. On Valve's track-pad-stick-replacement-things you would hold your thumb towards the top of the of the pad. If it's simulating a mouse it wouldn't keep moving up like a stick, it would just stop at that location. Sticks are an analog movement that can control the rate and direction of movement. A track pad that is moving as a mouse can't do that unless Valve has to specifically tell the controller games that need analog movement get analog movement. That way if you hold your thumb towards the top of the pad it will keep moving up, like a stick would. Then Valve has to tell the controller specifically which games need mouse like track pad movement so it doesn't control like a stick, like RTS games. It would be super complicated if Valve has to go through and tell each game how to see the controller.
Unfortunately I won't get hands on with one until I buy one. No one I know will buy one and I'm definitely not getting in the beta.