The Bush administration has wreaked havoc on the constitutional principle of separation of powers. The abuses of the executive branch have undercut both the legislative branch (signing statements, stonewalling subpoenas, and outright lawbreaking) and the judicial branch (suspension of habeas corpus, weakening of other constitutional protections, etc)
With that groundwork laid, what might happen with Obama (or a Democratic successor) in control of such an enormous amount of executive power?
Obviously, progressives are looking to Obama to clean up the abuses of the last eight years and somehow set everything right. It would be great to imagine that Congress will find some cojones and demand its proper place in the government, the Supreme Court will see the direction the wind is blowing and start issuing decisions limiting executive power, and that Obama will unilaterally decide to play fair... but what if it's for a really good cause?
Healthcare
With a Democrat in the oval office and Democrats in control of both houses of congress, we're likely to see some form of universal healthcare finally take its first faltering steps out of the gate. It probably won't take very much thinking for people to notice that many of the expenses that will be faced by the program are due to chronic health conditions related to lifestyle choices. Insurance companies get around that through denying coverage based on increasingly invasive probes of people's medical histories and personal habits. Congress has already considered bills to regulate cigarettes and trans fats. Imagine a government already empowered to spy on its citizens, facing a ballooning budget, and trying to contain healthcare costs... maybe a little too tin hat, but a lot of scary things have already been done in the past few years because it's for our own good.
Intellectual property
Democrats have counted on Hollywood for outspoken allies on human rights issues and for deep pocket campaign contributions. Hollywood has a vested interest in copyright protection and control over media delivery and creation. Republicans don't have a monopoly on bad public policy. With enough lobbyist pressure, there might be a clear path for some serious first amendment limiting legislation to fly through. Even so, do we dare believe that the signing statement pen might not come out if by some magic a libertarian bill guaranteeing internet neutrality or even watering down some of the more draconian parts of the DMCA manages to pass Congress?
We've come to see the jack-booted authoritarian side of the government as a recent Republican phenomenon, but much of the Bush administration's overreaching was first pioneered by Democrats-- Wilson and the Palmer Raids, FDR's rule by executive fiat and subsequent attempt to pack the Supreme Court in order to overcome contrary court decisions, Johnson and Vietnam from the Gulf of Tonkin incident to "turning a corner".
Our duty as citizens and especially in this election is to get people elected who will reign in the executive branch no matter who's in office. In some ways, an overwhelming Democratic majority in Congress and a McCain presidency might serve to goad the action we need, but however things stand this time next year we need to have made the first steps in fixing our broken government.