I think that this leftist disappointment is largely manufactured. Yes, Obama is a moderate, not a leftist, and many leftists are sad about that... but the man presented himself as a moderate all along. My belief is that the leftists turned out en-masse to support him for the same reasons that the vast middle (my whole thesis here is that most of us are moderates) turned out to support him. After GWB, any minimally competent moderate would represent a huge amount of "hope and change"
I think the set of people who voted for him that also later felt deceived by his policies is pretty small. I think that the talk of this feeling of betrayal and deception is largely manufactured by his political opponents; it's something I hear a lot from my friends on the right, and something I hear very little from my friends on the left.
The truth of the matter, as I see it, is that while he's not one of our best presidents, he's basically competent. He does his job, and after GWB, I think most people see a guy who can do the job with a basic degree of competence as pretty great.