Extreme4 versus Fatal1ty:
8+2 power phases versus 16+2.
4 SATA2 + 4 SATA3 versus 4 SATA2 + 6 SATA3.
1 GbE port versus 2 GbE ports.
0 PATA ports versus 1 PATA port (up to 2 drives).
4 USB3 ports versus 6 USB3 ports.
Blue and white color scheme versus red and black color scheme.
Theoretically, more phases equals smoother power to the CPU which would be better for overclocking. However this hasn't actually been proven. Any board with eight phases or more has the ability to overclock Sandy Bridge CPUs until they scream for mercy.