Good job, Democrats. You simply ran better campaigns than Republicans did and managed to run some middle-of-the-road candidates that sucked up Republican votes. Not only that, you got both houses of Congress and the White House, ensuring that you should be able to pass significant amounts of your agenda. Still, I have some warnings for you.
As of January 20, 2009, Bush is no longer the president. Most of your campaigning had to do with (successfully) tying Republicans to Bush and his policies. Once he's out of office, he is irrelevant. The moment you start talking about Bush anything, you're going to lose seats.
This was a continuation of the 2006 election. Seriously. This election was more about "throw the bums out" than a referendum on political ideologies. Don't sprint too hard to the left or you're going to get a repeat of 1994. This is especially important since you've been picking up a significant faction of conservative Democrats that could break ranks at the drop of a hat. Remember how poorly your leadership has corralled them since you gained control?
We all expect results. You got a free pass on doing nothing but passing Bush's favored legislation (FISA, anyone?) and failing to deliver on your 2006 campaign promises (remember ethics reform and Iraq?) by claiming that a veto was in your way. No more. You guys get something meaningful done or your name is mud. Peddling excuses about the minority party won't cut it; the Republicans didn't have much luck with that two years ago. It also doesn't help that you've pretty much promised that everything will be unicorn giggles once Bush and the Republicans are out of power. You set the bar very high for yourselves.
Make good on Obama's promise to work with the opposition. If you take a hard-line "we have the votes so screw you" position, you'll get more of the filibuster payback you've had to endure the last two years and be unable to get those results you need. This means not falling into the Republican trap of thinking you have a monopoly on good ideas.
I'm willing to wait and see if anything useful comes from the federal government over the next two years or so. Democrats, you have a tough job ahead.