"....it took NASA scientists (some of the best brains in the world) so long to understand just how powerful the system is."
It's not a question of 'dumping Windows' or of power - there will still be lots of Windows machines at NASA, and the routines that run on the space station itself are fairly low-tech in computing terms. Neither is it a totally new direction as far as the Space Station is concerned. Linux has been running on it since the day it was launched
The switch to Debian 6 is specifically so that the space station laptops can run an open-source operating system which NASA can adapt at will to suit the needs of the various scientific experiments that are conducted. These change at intervals, and NASA obviously can't start tinkering with the Windows code. Debian is known to be extremely reliable, and it will allow them to tweak and patch as they see fit, secure in the knowledge that if they break anything they can just start again.
Seems like a sensible approach to me.