My first thought was that having just a cross eliminates the ability to easily hit a diagonal on the D-Pad. I can't imagine it being easy than what was before.
The problem with having it be a disk is that it's hard to push any direction reliably. If it is a cross, if gives you fingers a guide, and actually makes it easier to go diagonally by doing a sort of roll from the top to side.
Personally I have an Xbox360 controller for use with my PC and both a Nintendo Wii and DS and I find that it's much easier for me to reliably press diagonals on the xbox controller, than either the DS or Wii controller.
Didn't Nintendo have a patent for the cross D-Pad? Hence all the other "fun" ideas from Sega, Sony and Microsoft.
Edit: Yep, the patent expired a while ago:
U.S. Patent 4,687,200 (expired in 2005) - Nintendo's multi-directional switch - http://en.wikipedia.org/wiki/D-pad#Patents