How to criticize Obama without being a racist: a four step guide from the Daily Kos, which is undergoing a series of "purges" due to accusations of racism against members who, three years later, are having second thoughts about Barack Obama.
Short answer: There are ways to criticize Obama without being a racist (as opposed to being called a racist), and there are negative things that one can say about the man that, objectively, are not racist. We're just not going to tell you what those are.
Do keep in mind that positive references based on Obama's race are generally acceptable. We don't have to always talk about Obama in non-racial terms. After all, whenever we speak of the "historic" nature of his Presidency, it's usually an explicit reference to the fact that he is the first black President. Clearly there's nothing wrong with that.
But even that can get dicey, however, because you open the door to a negative race-based criticism. Some have argued that President Obama cannot confront the Republicans the way many of us would like because he can't afford to fall into the "angry black man" trap. Well, OK, but does that mean that those of us who want a "fighter" for our side as President should never consider voting for a black man?
What happens when dissent arises in the midst of a community that, in large part, defines itself by ad hominem attacks on those who disagree with its political views as racist, fascist, and the like? A community of which a sizeable number are actively hostile to free speech as an ideal?
The results aren't pretty, but they're amusing.