Sean Trende explains its importance:

Of the 63 seats Republicans picked up in 2010, 15 were in Greater Appalachia. Without these seats, their majority would be much slimmer, and Democrats would be only 10 seats from a majority, rather than 25. Many of these seats represent districts that Bill Clinton had been competitive in during the 1996 elections, but which have slipped away from his party since as the Democrats have become increasingly liberal, urban and culturally cosmopolitan.

This is the real reason Tuesday night's results are significant.

Earlier commentary here.