Not only is he more "human" when debating vs his amazing oritorical abilities when delivering speaches, but it is also much more difficult for him to answer the much harder questions, that will be tossed around in the debates.
He will at some point in the debates have to answer questions about how you pafor universal health care, what raising taxes might mean to a shaky economy, why he has been opposed to drilling and what his plan for reducing energy prices would be, what the consequences may mean for pulling troops out of Iraq, etc.
His position sounds great at first glance, but doesn't sound nearly as good, when he'll have to have some semblance of reality.