Well it's only been around for a few years. It's just a mishmash of teams that were geographic anomalies in their old divisions. You've got the black-and-blue North, all the East teams are huge rivals, the West has the 49ers and Rams who at least used to matter, and then everyone else.
I still can't rattle off the AFC North and South teams, by the way. It's that Indy-Cincy thing that throws me every time.
EDIT:
New England over Denver
Indianapolis over Pittsburgh
Chicago over Carolina, again
Redskins over Seahawks, God knows how