The colors show 2008 winners. Groups to the left of the vertical reference line at zero that are blue switched winner from 2004 to 2008. All red groups also voted Republican in 2004, but most reduced the Republican margin.
Biggest Dem gains: Income over $200k, first time voters, Latinos, high school education only and age 18-29. The only three decreasing Dem groups were small towns and late deciders (though the late deciders still gave a majority to Obama.) And gays dropped their Dem support the most, by over 10 points though still favored Obama by a substantial margin. Fair warning that with only 4% of the national exit poll identifying themselves as gay/lesbian/bisexual/transgendered the sampling error here is pretty large.
While the labels are hard to read, a careful look replays some study. Click on the graph for a large size version.