I almost never play the American Empire either, though it isn't because of some weird aversion to Americans. It's because something about American cave-men working on Stonehenge seriously takes me out of the spirit of the game. It's sort of like having an X-Files marathon, and then during one episode, Bullwinkle flies in on a pterodactyl.
That said, I *ADORE* it when I speak to George Washington in Diplomacy -- his music is amazing.
With UBs and UUs, in addition to traits, I honestly feel as though a crack team of developers could account for every (or almost every) nation on Earth, and still make every leader a unique experience - even if, every so often, a UB, UU, or trait was similar to another culture's.
But then, I also don't think its very important to include everyone.
My people (The Swiss) were not represented in the game, and they could easily have been Financial and Charismatic, with a special bank UB that increased foreign trade and a special rifleman with increased defense in hills. But this adds very little to the overall experience, and since the game is so moddable, I imagine that the developers INTENDED for us to add the civs that we felt were missing.