Hands-down (pun intended), the design feature with the largest impact for me on the MacBook Pro has been the scrolling trackpad.
I have disabled the trackpad on every notebook computer I have owned. I was looking for the setting to disable it on the MBP when I found the “Use two fingers to scroll” setting on the same pane. I checked, tried it, and I will certainly not be uncomfortable without a mouse on the MacBook Pro again.
It shouldn’t surprise anyone that the company who brought us the solid state touch pad of the 3rd generation iPod scroll wheel (the one that didn’t have to move, it just sensed where your finger went) would bring an innovation like this.
Update: It drives me crazy now when I switch back over to the Dell to grab a few files that were left behind. I don’t have a mouse plugged in over there, so I use the track-pad. It drives me nuts when I drag two fingers down and it doesn’t scroll for me. I appreciate this new feature more and more each time I switch back to the old machine.