While reading about the Uruk-Hai, I couldn't help but notice how it is impressed upon the reader that they are far more evil than just Orcs.
Later it is revealed that they are half-breeds. A mixture of men and orcs. (I swear I read that somewhere but now I can't find the quote to save my life...)
In a land where each race is isolated from the other, either through distance or ancient feuds, could it be that the mixing of blood is what makes the Uruk-Hai so great a threat to Middle Earth?