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.
Somehow the party faithful needs to get the message that things could go right back to the way they were before if they don't remain vigilant about voting and staying involved with the process, but that's easier said than done.