For what it's worth, I think he's in a somewhat difficult position as the first African-American president. I believe, more than any other president in history, he is part of a specific constituency that looks to him to comment on a certain aspect of American life in a way that no other president has been asked to do. If he addresses race relations, then he has addressed those concerns, but possibly alienated those who think that he is targeting a specific group of people. If he doesn't address race relations, then he would not have alienated that group, but then he'd face a ton of criticism in both the African-American community and broader groups about failing to address race relations.
I think Obama is a good person, who hasn't been a particularly great president. In 100 years, he'll probably be remembered as the first African-American president, but won't be remembered in the same was that we remember Washington, Jefferson, Lincoln, Roosevelt, etc. To the extent he's been divisive, I think it's not necessarily something intrinsic to his nature, but more a reflection of our society at large, which has become more divisive and polarizing on political and social fronts. We don't want our leaders to be divisive, but we also don't want them to compromise on core issues, which unfortunately seem to be all issues.