
In circumcision, they remove the super thin layer of skin that covers the sensitive skin of the head. Why? Well, I'm Jewish, and it's a cultural thang, but for scientific reasons, it's supposed to prevent fungus from growing in the head (which modern soap and stuff prevents anyway), so it's mainly cosmetic today.
As for sensitivity, it's actually supposed to increase it, because without that layer of skin that separates the outside with the sensitive nerve skin, there's more friction with the head (come on Paul, when gettin' it on with LG, wouldn't you want her skin to be the one directly causing the sensation? Or do you want it to be yours?), and hence more sensitivity.
I actually think uncircumized would look freakish, your head would like a turtle head, in and out. Mine is fixed, always out for the ladies

And not all yanks are cut, I of course haven't done the research to find out, but I'd say more that 50% are not.