Tom Jensen has a good take on the enthusiasm gap between Democrats and Republicans:

When we talk about the enthusiasm gap there's generally an assumption that it's a product of Democrats who went out and voted for Obama but haven't been happy with the pace of change so they're now not going to vote this year.

I don't think that's quite right though- on our last national poll among the people who said they were only 'somewhat excited' about voting or 'not very excited' about voting Obama's approval was a 58/35 spread, much better than his overall numbers. Those folks also said they supported the health care bill by a 50/38 margin, again much better than we're seeing among all voters.

The enthusiasm gap may be caused not by disappointment with the way things are going, but rather contentment. Voters tend to get more energized when they're angry about something. A lot of Democrats feel like things are going fine right now, so they don't have much of a sense of urgency about going out to vote. The biggest threat to the party this fall is not that its voters are unenthusiastic about how things are going, but that they are complacent precisely because they do like the direction the country is headed in.

I think this is right. It's a dynamic that effects both parties to some extent, but I think it effects Democrats more. The Democratic base tends to lose interest in the threat of right-wing politics when their party holds power. Republicans, I'm guessing offhand, have had more success energizing their base during Republican rule. (Anybody want to quantify this?) Specifically I'm thinking of the 2002 and 2004 elections, which featured revved-up Republican bases despite total GOP control of government.

My seat of the pants analysis is that this reflects a psychological difference between the left and the right. The liberal coalition is more ideologically diffuse and attracted to individualism. Sometimes you see left-wing splintering at the end of periods of Democratic control -- 1948, 1968, 2000 -- but more often it's simply harder to make liberals understand the urgency of preserving their party's control of power against a hypothetical threat. Conservatives, by contrast, may find the idea of rallying behind a leader more attractive. Liberals were obviously very enthusiastic about the historical nature of Obama's election, but the enthusiasm has waned since. The conservative cult of personality around George W. Bush actually seemed to peak in 2004.