Ruy Teixera has a Prospect article that covers the familiar ground of why Election Day was carried by the GOP this year. He spends a few paragraphs on turnout, which should sound familiar:

Looking inside some of the Senate states the Democrats lost, preliminary analysis suggests that a couple of things were going on. First, while turnout across the United States was up from 1998 -- from 37.6 percent to 39.3 percent of the voting-age population, according to the Committee for the Study of the American Electorate -- it appears to be the case that in strongly Democratic large cities the increase in turnout was less than in strongly Republican areas in cities' outer suburbs or in rural areas. For example, in Missouri, the increase in votes cast from 1998 to 2002 was less in strongly Democratic St. Louis than in the strongly Republican suburb of St. Charles County or, especially, in rural and fervently Republican Cape Girardeau County. The same pattern can be seen in Minnesota where many of the more rural counties cast almost as many votes in 2002 as in 2000, while the more urban counties lagged behind.

But the other, and perhaps more important, part of the story was the reduction of Democratic support in Democratic-leaning suburban or mixed suburban-urban counties, where Republican success in picking off swing voters was likely to manifest itself. For example, in St. Louis County in Missouri, Jean Carnahan's margin over Jim Talent was only 3 points, down from the 8 points her late husband carried the county by in 2000. And in Hennepin and Ramsay counties in Minnesota, Walter Mondale's margins over Norm Coleman were substantially less than Mark Dayton's over Rod Grams in 2000 (11 and 10 points less, respectively). But in completely urban St. Louis city in Missouri, the Democratic margin was much the same as in 2000.

So it seems likely that a failure of core Democratic areas to match turnout increases in heavily Republican areas -- plus a shift away from the Democrats in Democratic-leaning suburbs -- were the factors responsible for many of the Democrats' key losses. Taken together, these trends meant that Democrats could not prevent highly-mobilized Republican areas from dominating these electoral contests.