The states below the 45 degree line swung toward the GOP -- most notably Arkansas and Louisiana. Arkansas is probably still undergoing the process of de-Clintonization and is rejoining the rest of the South. Hurricane Katrina ethnically cleansed some of the poor blacks living below sea level in New Orleans, so the state has moved to the right. I'm not sure what's going on in Oklahoma. The GOP did well in Alaska, presumably due to Sarah Palin. The GOP also did well in Massachusetts due to a favorite son not being the Democratic nominee.
The real outlier for the Democrats is Hawaii, which is, presumably, a favorite son effect for Obama. Vermont is just becoming Vermontier, Delaware is undergoing a long term shift from being a bellwether purple state to a solidly blue one, Nevada was driven left by the Hispanic influx and the highest rate of foreclosures in the country. I don't know why Indiana jumped so much to the left -- perhaps Gary and the rest of Greater Chicago in Indiana was fired up for the local hero. Utah's move from being ultra-Republican in 2004 to just highly Republican might have something to do with growing Hispanic presence, or from Mormons being sore at McCain beating Romney. (Nine months ago, I wondered how Mormons would react to the anti-Mormon animus seen in some of the GOP primaries.)
Anyway, you can see the new best fit line would simply have shifted up (in the Democrats' direction) a few points, with a pretty good fit.
And here's the quivalent graph comparing 2000 to 2004, with the 45 degree line representing how Gore did in 2000. A very, very similar pair of elections, just with Bush running about 3 points better in 2004 than in 2000 almost everywhere in the country.
If you drew up the equivalent graph for the 1952 and 1956, which featured Eisenhower and Stevenson running both times, it would look more like a random scatterplot. On a state-by-state basis, the political environment was a lot more dynamic in the 1950s than today.