Silver helps us understand it:

The enthusiasm gap has more to do with abnormally high levels of Republican interest in the election than with despondent Democrats. ... stories that cite the enthusiasm gap as evidence of malaise among the Democratic base are probably miswritten. Such stories may conflate the priorities of certain subsections of the Democrats’ broad constituency or of some especially vocal activists with those of Democratic voters writ large, most of whom are focused on the poor economy and the government’s efforts to respond to it, as voters of all political persuasions are. Such analyses may also ignore the history of midterm elections, which suggest that Democrats usually do suffer from an “enthusiasm gap” of some kind. Finally, they may be burying the lead, which is the unprecedented level of political engagement by Republicans this year.