Rarity is like obscenity: I know it when I see it.
I'd entirely omit condition rarity from the discussion because it is based on assumptions about a pool of cards [those cards that have been slabbed] that is not a comprehensive measure of all cards out there and that I don't think is all that objective in the first place. I tend to look for items that simply aren't found with any regularity. I saw this at the 2011 National, passed on it, kicked myself all year, then stumbled across it again and bought it in Baltimore:
That's rare. It is also obscure, like this one [European seller of a set over Ebay of late notwithstanding]:
I find those items infinitelu more interesting to collect than one of these, even though it sells for more:
Boring to me, catnip to others. That's the beauty of collecting.