As I've noted previously, I think this is a pretty valid critique of the Obama campaign.
However, I don't think it is completely true that Obama has not taken questions from voters at particular events. Being on the campaign's email list, I received a video clip of Obama appearing to participate in just such a forum and answering a question--this one in particular being one in which the candidate was asked to justify his candidacy relative to Hillary Clinton's.
I can't say how regular this is. And I can't verify the degree to which this forum or question was staged or not.
Nonetheless, it would seem Obama would need to develop more of an accessible campaign if he gets the nomination.