What has he done?
Why do people consider Obama as a bad President?
Because of the national debt mostly, he's been neglecting it. However Obama's biggest success is probably that he speaks for Muslim rights. Or foreign oil dependence down, but Obama's national debt is atrocious.
One reason simply because he's the president and people need something to talk about. Another is that people don't agree with some of his policies or they see him as inactive.
The man himself is not bad per say. However his place in history, being the first black man ever elected to the presidency of the United States, has resulted in the resurgence of many negative groups and organizations which use his ethnicity to perpetuate the ridicules falsehoods which surround him to discredit him as well as the US Government. On the other hand he has made several decisions which have caused major rifts within the social, moral and economic fabrics of the United States.
The essence of the United States has been changed and there is no "going back to the good old days". Away from the public's eyes and ears there is an ongoing real time struggle / battle within the country to put it back on a strong solid course. The alternative would result in a massive change, so negative it would cause the world to suffer as no depression or war has in the past.
Well, just speaking for myself - his health care law (affordable care act) was supposed to make things better, but it made my health insurance cost more than ever and I now get less benefits. The plans I had before his law are not even available now, the company had to cancel them.