Actually coordination is a part of one's athletic ability imo. He just had the quickest hands, amazing hand eye coordination, really coordinated for his size, great footwork, deceptively quick also especially in small space, great stamina, really strong and relatively mobile for his size.
He didn't have below average athleticism, especially in his younger, non-injury years, dude was a pro-athlete, you can't be a pro bball player with below average athleticism. He was decent in terms of athleticism, especially when considering those things I mentioned above, when compared to other NBA players, especially the most athletic ones, I would agree about the average athleticism, but not really below average though.
Yea, seems like Bird has some haters around, I don't know about the black thing, I'm black, well, white father and black mother, don't know if that's what changes things, but I know black guys who really admire his game so I don't know.
Maybe it causes hate to see a white hick dominating in a sport where the best players are usually black, or like you've said, a dude that's not really athletic, making athletic freaks look like fools, say like this: http://www.youtube.com/watch?v=qycH9GYDK74
And that's just one example out of many
edit: Also found this: http://www.youtube.com/watch?v=ZVvmmiPA4is&feature=plcp
A guy with below average athleticism can't do that. He also has the same vert has Karl Malone, and Karl was never called a non leaping guy, probably because he's black and has that crazy frame.