A reader points out that for the Democrats, the most obvious factor is that younger Democratic men really don't like Hillary. Otherwise, nothing much jumps out at you. They certainly don't seem to differ due to any issues.
The reader adds:
The Republicans could be called the Cancer Kids, since three of them are battling cancer -- McCain, Giuliani, and Thompson. Maybe that's the issue that will finally get Romney out of his second place rut -- "I'm probably cancer free!"
The Dem side was driven by who you are while the GOP was by what you believe. Note that the largest divergences between Romney and McCain were on Iraq, the economy and immigration. Under the surface, it appears Romney was the de facto surrogate for GWB.
My favorite stat: Among voters under 24, Ron Paul beat Hillary (different sets of course, but you get the point).