This is not some rule I’m coming up with at this moment. To my understanding, black people are very sensitive to white people being patronizing of them, and are annoyed when white people give them compliments that can fairly be considered slight. It suggests a lack of equality between the two, in which the recognition by a “superior” party of a slight accomplishment is the best that can be expected of the “inferior” party.
To say that Obama, who at the very least is one of the great orators of our time, is merely well-spoken or articulate, as though that represents the height of his potential, is to give short shrift to the tremendous accomplishments he has achieved in other areas and all the hard work he put in to get there. It’s not unlike when they used to say that Willie Mays is a great ballplayer because of his uncanny instincts and his great speed. Sure, he had those, but to imply that Willie’s instincts and speed were the sum total of his greatness sets both the floor (the expectation) and the ceiling (the ultimate compliment) too low.
Again, this is my understanding of it, and ironically, I may not have articulated it exactly as it could have been, but I think I’m at least pretty close. I know some people here will harshly disagree. But it makes sense to me.