The map from 2004 was unquestionably much more red.  Many seats went uncontested by Democrats.  (Note the scale:  All seats with >70% Republican vote, which includes uncontested races, have the same color.)  

Click to enlarge.

Comparing the two maps shows a combination of two success stories:  running more candidates, and increasing the Democratic support in the vast majority of districts.
Below, the change in percent votes cast for Republicans.  The most drastic changes, again, are the result of one or the other party not running a candidate in either 2004 or 2006.  That being said, look at this solid sea of blue:

Click to enlarge.  Close-ups here of northeast and northcentral regions.

These moral victories are just the icing on the cake of the real victories we saw last week.  They represent real progress in bringing voters to the Democratic side.  

Here are two more maps:  The state-by-state average share of the House election vote that Republicans received in 2004 and 2006.

Click to enlarge.

These maps include the small urban districts and give a little better idea of the general support in each state, but the numbers are still skewed by uncontested seats.  In Massachusetts, for example, the Republicans seem to have pretty much given up on running for the house, which is why the state is at such an extreme.

To avoid that problem, the map below shows the change in each state's Republican support, but calculated only using districts that were contested in both 2004 and 2006.  This map show the state-by-state change in voter sentiment pretty well (except for Massachusetts, where only one seat was contested both years.)

Click to enlarge.

An overwhelming shift, across the country, except parts of the Deep South.  But even there, Howard Dean's nose-pickers in Mississippi, at least, must have done a good job.  There's also less of a shift evident in the urban West Coast and Northeast Corridor, where support for Democrats was already high.

Some other interesting things to point out:

Republicans gained in Vermont (20 points) - where Bernie Sanders left his seat - Louisiana (4 points) - where Katrina displaced Democrats - and Georgia (1 point) - where there was redistricting.  But nowhere else.  

Democratic incumbents in the Dakotas far outperformed themselves compared to 2004.

That's the graphic House summary on a national level.  If you hunger for more, you can find a ongoing series with region-by-region analysis, and, yes, lots of fun maps in ManfromMiddltown's diaries. And, of course, here is an excellent national summary of all the 2006 elections by johnnygunn.

Numerical Tidbits

Republicans won by a margin of less than 10 in 29 districts; Demcrats, 25; Republicans won by a margin of less than 20 in 74 districts; Democrats, only 37.   (Numbers may change with final results.)

The percent of voters voting for Republicans decreased by an average of 3 points in all Democratic-held districts, and 7 points in all Republican-held districts.  In the Republican-held districts displayed in the DailyKos watchlist at the top of the page, Republicans lost on average 9 points of support.  These numbers are for races that were contested in both 2004 and 2006.

Take-home lesson

Johnnygunn showed us the real victories.  This diary focuses on all gains, whether they resulted in a victory or not.  Democrats are seriously pushing back against Republicans across the country, even some states in the South, like Mississippi.