In 2006, after being in the minority for a decade, Democrats re-took the House. Democratic candidates won 30 seats that had been held by the opposition in the previous contest.
As of January 2015, only seven of those 30 Democrats will still be in the U.S. House of Representatives.
We reviewed that Class of 2006, looking at how they did over the past five cycles to put together the chart below. The bars are scaled to margin of victory; when and if the bar turns red, that's when the Democrat lost. The "retirements" listed are for a variety of reasons, including running for office (Braley), stepping up to the Senate (Gillibrand), and plain old retirement, etc.
Since none of you are going to scroll to the bottom of the chart, a key point: Rep. Carol Shea-Porter (D-N.H.) won in 2006 and 2008, lost in 2010, won in 2012, lost to the same guy in 2014. When people talk about the see-saw nature of the new political landscape, Shea-Porter is about as good an example of what's being talked about as you'll find.
Update: It's been pointed out that counting the seat in Vermont, a pick-up from an independent, not a Republican, is a bit iffy. Particularly since the independent at issue is now-Sen. Bernie Sanders (I-Vt.), who is hardly a Republican. So, there's a grain of salt for you.