In my opinion (which may be highly distorted because I am not US citizen) the main problem Obama had is the hype created on his first election campaign. People thought he will revolutionize the country but the changes he achieved were not that big. Nobel Prize in 2009 also generated more hype.
I also think it is very soon for appreciate his legacy. I think Trump will be a savior from Obama legacy, but I hope I am wrong.
While I do agree much of his administration was hyped up, he did deliver on many of his promises. Obamacare helped put millions of people on insurance plans and he took us out of Iraq. The promise of change he made was not an overall change in American society, but instead a departure from the conversativism of Bush. Unfortuantly Trump is now destroying Obamas legacy by activaly trying to kill Obama care and taking down many of the enviromental regulations that Obama put in place.
I believe than a public healthcare system is much more efficient than a private one, I am thankful I live in a country with one of the best public healthcare systems (Spain) in both quality and cost-effective, and I have noticed that all attempts for privatizing increased the costs and reduced the quality. So I think Obamacare is a great idea. I know it is not a health nationalization program at all, but it is a small advance in that direction.
I think Obama certainly had the goodwill of being a much more peaceful and diplomatic president than Bush, but the circumstances did not allowed him to do that. I am not comparing Bush and Obama military policies, Libya and Irak operations are very different, and it is very soon to value interventions in Syria.