Prior to politics, Obama was a constitutional scholar.
I'm aware, he was a lecturer, not a professor and hardly a constitutional scholar. He never published a single Law paper, ever. He was hired directly out of Harvard Law school based on the notion that he'd accepted a book contract to write about Race and Law, he reneged on this contract and never wrote the book. He eventually changed it from a Law book about law and race to his memoirs which he also could not finish. He eventually had Bill Ayres (yes the terrorist Bill Ayres) write the book for him.
He as not a constitutional scholar, he taught a course about race relating to the constitution. That is quite different altogether. All of his courses revolved around race, rights and gender. None were specifically focused on the constitution alone, which would be what a constitutional scholar would lecture about.
He didn't teach about the Constitution, he taught about what he wanted to fundamentally change about it. He taught about historical race relations, gender relations etc. He did NOT teach about the constitution, why it was put together the way that it was and how we can use it to interpret case law today. No, he taught about how he doesn't believe in the legitimacy of it, why it needed revision in his opinion.
In other words, he would be terrible for SCOTUS because he doesn't believe in the document that is the basis for all SCOTUS arguments and cases. He wouldn't fit in, he wouldn't do the job correctly. He would be the definition of an activist radical judge who wants to write law from the bench.
http://www.nytimes.com/2008/07/30/us/politics/30law.html?pagewanted=all&_r=0
He's been on radio speaking quite openly about how much he despises the Constitution at a base level because it doesn't have anything in it about redistribution of wealth. https://www.youtube.com/watch?v=sMJS4CP3OC0
This guy would be a truly terrible choice, at least Sotamayor takes the court and its role seriously and has some respect for the document that she uses to interpret law.