In the vein of Are inter-racial marriages frowned upon? - Pete Hines, Bethesda's VP of PR and Marketing, when asked if Skyrim would support gay marriage said the following:
You can marry anyone...Not any NPC, just mean any male or female.
So obviously some NPCs are off the list. I assume I can't marry Jarl Balgruuf, but what about Irileth? What about the Graybeards? What about already-married NPCs? Is it any "important" NPC, or is it more or less than that?
Has the Skyrim Creation Kit shed any light on how to determine this?