Joe Stiglitz is a Nobel Prize winner in the field of economics (he won the award in 2001), he is currently a professor at Columbia University, and he is making the rounds on the media circuit in support of his latest book, The Price of Inequality. (You can read an excerpt from the book here, which is probably helpful since much of the interview is about the European financial crisis and such.)
My guess is that Bill Maher is predisposed to like Stiglitz because Stiglitz is in line with Maher's liberal ideology, but, in reality, Stiglitz is much more objective and practical than Maher would probably like to believe. For example, Stiglitz's argument isn't really that inequality is bad on principle, it is instead that inequality has its downsides even for those who are at the top of the wealth pile. (In fact, the Vanity Fair article linked to above contains the statement "Put sentiment aside.") This divide makes the dynamic between Maher and Stiglitz pretty entertaining.