I think Joe Klein has the most succinct (and, not coincidentally, the most dour) assessment I've read:
Obama . . . entered the primary as a fresh breeze and left it stale, battered and embittered — still the mathematical favorite for the nomination but no longer the darling of his party. In the course of six weeks, the American people learned that he was a member of a church whose pastor gave angry, anti-American sermons, that he was "friendly" with an American terrorist who had bombed buildings during the Vietnam era, and that he seemed to look on the ceremonies of working-class life — bowling, hunting, churchgoing and the fervent consumption of greasy food — as his anthropologist mother might have, with a mixture of cool detachment and utter bemusement. All of which deepened the skepticism that Caucasians, especially those without a college degree, had about a young, inexperienced African-American guy with an Islamic-sounding name and a highfalutin fluency with language. And worse, it raised questions among the elders of the party about Obama's ability to hold on to crucial Rust Belt bastions like Pennsylvania, Michigan and New Jersey in the general election — and to add long-suffering Ohio to the Democratic column.