Barack Obama takes the oath of office today amid a sense of expectation and opportunity rare even for new Presidents. Partly this is due to his heritage and the historic nature of his triumph, partly to our current economic troubles, and partly to a nation looking for a fresh start after the difficulties of the Bush era. The paradox is that in order to succeed Mr. Obama will soon need to turn the opacity of his hope into clear and often difficult choices, some of which will upset his most passionate supporters.