First, he's a politician. If he had been too blatantly left-wing he wouldn't have been elected by a population which has less than 20% who self-identify as liberal. I think this explains his opposition to gay marriage and his declaration that the abortion issue was "above his pay grade." He's savvy enough to pick his battles… usually.
Second, he's a terrible leader. The man may be a condescending, Ivy-League intellectual who has strong socialist and revolutionary ideological roots… but he can't make the Washington machine actually do anything. His liberal credentials were great – and in 2008 he captured the imagination of a lot people for a lot of reasons – but he's been completely ineffective, even when his party controlled both houses of congress. What excuse does he have other than ineptitude?
So now, I'm hearing the buzz from all corners saying that the President has disappointed liberals. Okay. Here's my suggestion: Run a third-party candidate. If the Democrats are too moderate for you, go get a real revolutionary like Cynthia McKinney or even Al Gore or some other left-wing whack-a-doodle and run them for president against your own guy. That's how conservative purists
Go for it!