Matt Yglesias linked to an old Jon Chait article called "The Case Against New Ideas," written in the wake of the 2004 election. It indicates that Democrats don't necessarily need to spend time coming up with new proposals -- they already have tons of them -- they simply need power. He says, "[Lack of media coverage on Democrats' ideas] isn't because reporters harbor a bias against liberals. It's because they harbor a bias against ideas that stand no chance of being enacted." This was especially true in 2004, and now it's changing a bit.
The simple fact of the matter is that getting things passed is hard, especially with a divided government and the power of nearly any given lobby. Even when a party gets an idea through the House and Senate, it's usually altered -- sometimes for the better and sometimes for the worse.