I don't really think that a casual gamer would like Uncharted... They can't complete it simply by waving a remote around to play baseball or touch the screen to pet a dog so it's probably way too hard for them.
Anyway, I say 5, right in the middle of the difficulty scale. Finding the treasures isn't too hard since they are pretty shiny for you to see easily, and playing Crushing isn't that hard after you went through Hard mode (as long as you didn't cheat your ass off in hard mode, anyway). But there is still a decent level of challenge. You won't be given free trophies. You have to earn them.