I wasn't too sure about the touch pad at first, but I've given it a bit of though over the past few months and think it could be pretty cool if used right. For example, I'm not sure how sensitive it can be to where your thumbs come down on it or if different gestures could be used in game. If so, what if each thumb had a few basic gestures (swipe up, down, left, right, slow drags, circles, etc...) This could be really good for games with lots of items and powers to use (MMOs.)
The one thing that I think would have been better would have been if the pad were on the back (bottom?) of the controller by my middle fingers. I thought of this when considering the Vita rear touch screen, which is kinda not so useful. I'd have rather had some buttons back there... and it's difficult to not touch the back bad sometimes. But on the Dualshock, I think the rear touch pad would have felt much more natural and removed the need to let go of the sticks.
Just a few thoughts. What does everyone else think about the touch pad? How would you like to see it used? And if you could change something on the DS4, what would it be?