One of the main reasons behind Obama's success is intelligence, which is lacking in Bush. His campaign won the votes of two groups from the very beginning - African-Americans and the youth. He always encouraged young people, who are not very much into politics, to vote and take part. They represent a large portion of the American population, so winning such a group is a very intelligent move, Mr. President.
Luck has played a major role in Obama's success. Luck? Yes, it is luck that very much contributed to Obama's win. One of the main points of strength in the Republican McCain's campaign is war against terrorism. No terrorist attacks have happened in the past months, so Obama kept support and people didn't rush to McCain seeking protection. Such events could really shift consequences, but for Obama's luck, it just didn't happen.