Many people see Obama's failings to date as a president, and his inability to achieve real, progressive change as evidence that he actually isn't a radical leftist, as many people believed him to be during his campaign. I would argue that his values haven't changed much, if at all, from the time when he was the most liberal member of the Senate. The realities of the Constitution and this nation's (blessed) dispersal of powers, our checks and balances, are that they can prevent a radical socialist from changing the country's foundations even when he occupies the most powerful position in the land.
So what's the danger of having a radical ideologue as president then? Well, even though Obama has proven to be impotent, thanks to the other branches of government, and a (blessedly) stubborn, uncooperative opposition party (as well as some moderate members of his own party), things still hang in the balance. He's perilously close to being able to do literally anything that he wants. And if he could, please believe that he would. Progressives calling for him to simply take charge and ram his agenda through are being naive. Don't you believe he wants to do exactly that? It just isn't that simple. On health care reform, triangulation is absolutely necessary to get literally anything done at all. He has to compromise to get it through Congress. It's just a cold reality that he has to deal with. But do you think if our system was such that he could just sign an executive order and get it into law right now, despite opposition from the majority of the country, he wouldn't do it? When you see him threatening to effectively overturn decisions of the Supreme Court in his State of the Union address, in front of the entire nation, with the Court seated in the audience as his guests, it should be quite obvious he has little respect for other branches of government, or for our system as a whole.
This is why our Constitution is the greatest political document ever written. It can take a radical progressive, put him in the nation's highest office, and turn him into an ineffectual pragmatist. God bless America's founding fathers and founding document.