Biden's restraint
One of Biden's goals going into the debate was to show restraint, something he seems to clearly be showing. He is sticking to the facts, and attacking McCain and Bush but rarely Palin.
Again, and I cannot say this enough, Palin is hanging in there. If she survives this debate without making any major gaffes, that will likely dominate the media narrative going out of the debate (which is probably more important then the content of the debate itself.)