The Hotline provides an excellent analysis. Essentially it came down to the outside-the-beltway Democratic Party making its efforts work, and the DCCC and SDCC figuring out how to take advantage of those efforts in the last minutes before the election when Schumer and Emanuel begain to realize what was happening. The key was when the State Democratic Chairpersons elected Howard Dean to the Chair of the Democratic National Committee and the Netroots began to have local effects and to prove they could raise money.
Go read the article. It will give you a much better idea what was really happening in the Democratic Party. Then consider - this was only the beginning.
[H/T to Christy Hardin Smith at FireDogLake.]