There's a temptation for progressives to entertain Ron Paul as a serious alternative to Obama, primarily because Ron Paul is very, very serious about getting rid of a couple of major federal programs that progressives tend to hate: namely, our War on Some People's Terror and our War on Some People Who Use Some Drugs.
The problem, as Ben Adler points out, is that Ron Paul's motivation for opposing these programs has nothing to do with the progressive motivation for opposing them. Most charitably, Paul just cares about limiting federal power. His administration would care little about the impact of federal policies on various populations; it would only care that the government pursued those programs at all. This means that the end of the War on Drugs would come alongside a push to end Medicare and Social Security, a push to end all forms of social welfare, a push to end everything designed to ameliorate the effects of systemic discrimination over past decades and centuries.
Ron Paul doesn't care about equality or social progress, he's just an adorable shrunken grump who has an ideological opposition to the government doing most anything. That opposition has certain incidental benefits, and it's hard not to think of him as a useful tool in achieving long-term political goals.
Less charitably (and, I think, more honestly), Ron Paul by and large only gives a shit about maximizing the freedom of white men. The War on Drugs is problematic not because it helps incarcerate truly ridiculous numbers of young black and Hispanic men, it's problematic because white guys deserve a doobie or some blow after work. The War on Terror is an outward extension of American resources and manpower, but the person whose freedom we care about isn't the little girl disfigured by a drone or the imam whose mosque was destroyed. It's the white guy who works long, hard hours to pay for that war, who would much rather be spending his money on other things, like gold bricks or gold boullion or ads trying to get people to buy his stock of gold.
What that ultimately means, though, is that the shining moments of a Paul presidency would be largely flash. Paul's libertarianism would mean an end to the War on Drugs, but it would also mean an end to enforcement of the Voting Rights Act and the Civil Rights Act, a push toward a future of rampant Tentherism where a state's discriminatory action would be met with a shrug and a casual bon mot about the Fed.
The appeal of Ron Paul is that he comes off as truly principled. Even when his policies may achieve a goal of racial equalization that he would seem to be otherwise opposed to, you're still assured that he'll advocate for those policies.
That allure, however, masks the dirty secret of his appeal to progressives: we're so sure that he'll pursue the policies that we like, we might be willing to compromise on the other stuff. The problem is, that other stuff is the very core of progressivism. The scant victories a Paul presidency promises are meaningless when they're the curtain hiding the abyss.
Unless, of course, you have a whole lot of gold. At that point, I can't really blame you.