The first is that she has gained tremendous, even unparalleled experience, at the United Nations. She has learned how to parry the Russians and the Chinese; she has figured out the snakepit ways of the international system; she has seen up-close the hypocrisy of totalitarian and anti-democratic states (states that still make up a good portion of the UN membership). At the UN, Rice has become an eloquent voice for human rights, and she has done an able job of arguing against the wildly disproportionate criticism leveled at Israel in the General Assembly and in putative UN human rights forums. She has been far from perfect in the job, but she has generally been solid.Reason #2 is the most compelling. An old saying is Never pick a fight with anyone showing scar tissue. The pages of history list many examples of failure, some of whom sank from sight while others made a comeback. But there are few examples of highly successful people without failures in their past.
The second reason: She has had some very public failures. A secretary of state nominee -- anyone in high office, really -- should have some experience with failure, and she has it, most notably on Rwanda, during her service as an Africa expert in the Clinton Administration. She realized soon after the genocide that her Administration was derelict and absent from the scene, and she has spoken movingly and with apparent sincerity about her own shortcomings.
The third reason is related to the second reason: For people who believe that America has a benevolent and positive role to play in the world, in confronting dictators, stopping genocide and highlighting human rights abuses, Rice should be their candidate. For isolationists, Rice at State would be a real challenge. She is inclined toward humanitarian intervention -- I believe she had these inclinations even before she saw the price of timidity and inaction in Rwanda -- and her active stance on the Libya intervention (and the obvious tension she feels about the so-far limited role the U.S. has played in Syria), suggests that she won't be afraid to recommend to President Obama greater involvement in the world's crisis zones. (One of the reasons John McCain's operatic opposition to Rice's potential nomination makes no sense to me is that he shares many of the same activist inclinations as Rice)