First and foremost, he is a black man who has achieved the highest office in the land. Racism runs deep and strong in America. Even those who do not openly display their dark, deep and secret racism, allow it to manifest in other ways.
Second; while he is little more than "Republican light" he has rubbed against the grain by being vocal about giving a shit about the common people of this nation and has upset corporate America and the elite by doing so.
Third, He is the personification of change (not nearly enough albeit) and Americans (especially these dried up, bitter, old white men of the baby boom)don`t handle change well.
Fourth, he has a radically (to some) different philosophy about international relations; especially when it comes to the Islamic world. Americans have been brainwashed to be fearfull of everything we don`t understand or does not look exactly like us. We are not longer the "home of the brave" we have become a nation of cowards, hiding behind a big gun called our Military. Obama represents risk and an element of bravery that shames and threatens many cowardly Americans.
The list goes on, but I see that as the main points that the haters rally around.