The real Barack Obama is still unknown, supposedly.  One take is that he is a smart guy who will move toward the center and govern moderately and competently.  Another take is that he is the most radical person to ever hold the position of President who will take the US on a hard left course into a socialist state lacking any resemblance to the country our founders gave us.

• Gays in the military.  Congress could change US Code, Title 10, Section 654  to allow open gays in the military.  (Congress sets such rules.  President Clinton tried to change them by Executive Order, but the courts ruled only Congress can change its own rules on military regulations.)
• Gay marriage.  The Democrats could overturn the Defense of Marriage Act and thus make gay marriage the law of the land throughout the country, including how such couples are treated on federal taxes and Social Security rules.  That is, since gay marriage is already the law in at least two states (by court rulings, not legislation or popular support), ending DOMA would force recognition of those marriages throughout the states and federal government.
• Stem cell research.  Federal funding of research that destroys human embryos, beyond a few existing embryos, was restricted by Executive Order under President Bush.  President Obama can eliminate this Order immediately, and Congress could then get to work passing legislation to fund human embryonic stem cell research, thus incentivizing early abortions that yield such embryos.
• Abortion.  Congress could pass the Freedom of Choice Act and overturn the Hyde Amendment, and thus eliminate all restrictions on abortion and get the federal government back into the business of funding abortions in the US and around the world.
• Censorship.  By bringing back the so-called fairness doctrine, conservative voices such as Rush Limbaugh, Sean Hannity and virtually all of talk radio would be eliminated.  The internet could be regulated and political speech further restricted under McCain-Feingold type rules.
• Mandatory national service.  Obama stated that national service would be the "central cause" of his Presidency.  At a minimum, this would likely entail dramatic expansion of AmeriCorps and the Peace Corps as well as mandatory "public service" hours as part of high school and college requirements.  It could mean a draft, or mandatory service, and a national force in size and funding comparable to the current US military.  Think of it as ACORN with $500 billion in annual funding and that your sons and daughters must serve.
• Socialized health care.  While I have stated tepid support for some kind of universal health coverage (with significant caveats), what we will likely get from the all-Democrat federal government will be full-blooded socialized medicine from price controls on drug companies to insurance mandates and a Medicare-like bureaucracy imposed throughout the system, with no simplification of or reductions in Medicare and Medicaid.  A single-payer system like Canada's or the UK's will look good by comparison.
• Court appointments.  President Obama will nominate, and the Senate will approve, the most leftist, "living-breathing Constitution", judges to federal courts.  Such judges could easily rule that the right to keep and bear arms is not an individual right; that gay marriage is a Constitutional right; that women must receive comparable pay for like work; that the 14^th Amendment's equal treatment clause requires taxpayer funding of just about anything you can think of, from schools to sports programs; that the Boy Scouts must allow openly gay adults to take 12-year-old boys camping; that school administrators have more say in your child's upbringing than you do; that "private property" is effectively a meaningless concept if you do not use it to serve "the public purpose" as conceived by any public official; etc.  (Here's a secret: courts have already ruled in favor of many of these concepts.)
• Taxes, spending, regulation.  Taxes will be increased, especially on people who pay them.  Social Security taxes will be applied to all income levels.  The Death Tax will be restored in full, and then some.  We might get totally new taxes, such as a national sales tax, a value added tax, or a wealth tax.  Spending will go up, perhaps dramatically, on everything but Defense.  New regulations will be imposed on everything from investment banking and hedge funds to work rules and insurance mandates.
• Foreign policy.  It will be all talk and no action when it comes to national defense.  We will reward enemies (e.g., Venezuela, Cuba, Russia) and punish friends (e.g., Columbia, Israel).  We will work through the UN as much as possible, and we'll probably put ourselves under the jurisdiction of the World Court.  Look for Donald Rumsfeld and perhaps George W. Bush to be prosecuted in the Hague.
• Rule of law.  Voting rules will be weakened so that vote fraud will be widespread but undetected.  The border will become a meaningless line on a map, since border enforcement will be weakened, no one will be considered "illegal", and every border-crosser will get all the rights and benefits of citizenship once foot is set here.  Generally, think Chicago-Daley on a national scale.