So it looks like Obama vs McCain. This should be an interesting race. Both candidates are essentially running without the whole hearted support of significant portions of their party's traditional coalitions. Let's look at some of these groups. Keep in mind that people have three choices; Obama, McCain, or Stay Home. Keeping that in mind, let's look at some of the groups that have been talked about.
African-Americans: They will vote 95%+ for Obama and will turn out in much greater numbers than before. In some places, New York for example, it won't change the electoral map at all, it will merely pad Obama's margin of victory. In some places, like Virginia, it may make a significant difference.
Hispanics: Obama had difficulty with this group in the primary, Bush did better with them than any previous Republican. McCain does well with them in Arizona. I think this may the election that tips Hisanics more firmly into the Republican camp. Immigration will be an issue that can spoil this though. All bets are off if Obama picks a Hispanic Veep.
White working class: This is the group that matters. Obama did not do well with them at all in the primaries. McCain's war record will play well. Many of these people were in their teens and twenties in the 80s and were the young Reagan Republicans. McCain's no Reagan, but this is one group that won't have a problem voting Republican.
Boomer Women. These are the "Soccer Moms" of 12 years ago. A lot of them wanted Hilliary and wanted her bad. These are the people that think that Hillary was subjected to all sorts of sexist outrages. Some will consider McCain, and might even vote for him. Some will, with reluctance, vote for Obama. Many will stay home. This is a problem for Obama, as they were a vital part of the coalition that elected Bill, and kept Kerry and Gore competitive.
Younger voters: Will go for Obama in big numbers in some parts of the country. But will less of a factor than commonly supposed.
Conservatives: Some will hold their nose and vote for McCain. Some will stay home (or vote third arty) to teach the Republicans a lesson. I think the election will depend on this ratio. If enough conservatives opt out, McCain is dead.