Storage performance
Disk speed is hugely impressive for a laptop, thanks to Rock RAIDing a couple of nippy 60GB 7,200RPM drives. You can't really discern the difference between, say, Pentium 4 3.2GHz and 3.6GHz processors, but a speedy disk subsystem is immediately appreciable. It makes sense to increase the performance of the slowest part of any PC.
USB 2.0 performance isn't too shabby.