The low turnout is what makes it so unpredictable - that, and the new context.  The context was now that of an overwhelming Democratic victory in the Presidential and congressional elections.  Did voters want to reinforce that even more, or exert some balancing restraint - especially in a state which is very conservative/Republican in the first place.  Were Democrats sated or still enthused?  Republicans alarmed or despondent? Martin didn't seem to enthuse anyone whilst Chambliss could present himself as an independent voice of restraint on the wilder excesses of a still unknown President/liberal congress.  If Obama couldn't be bothered to turn out, why should his supporters?

notes from no w here