Her excellent work as first lady. IMHO, her experience as a first lady MATTERS. Why? Because she didn't just sit around doing nothing, she was ACTIVE in political and administrative fields. There's no doubt that she's learned a A LOT from eight years of that. She also demonstrates her energy, productivity and big heart. She is a proven leader.
Some highlights:
--Hillary Clinton was the only First Lady to keep an office in the West Wing
--The First Lady was also one of the few international figures at the time who spoke out against the treatment of Afghani women by Islamist fundamentalist Taliban that had seized control of Afghanistan
--One of the programs she helped create was Vital Voices, a U.S.-sponsored initiative to promote the participation of international women in their nation's political process. One result of the group's meetings, in Northern Ireland , was drawing together women leaders of various political factions that supported the Good Friday peace agreement that brought peace to that nation long at civil war.
Read more about her biography as first lady at
http://www.firstladies.org/biographies/firstladies.aspx?biography=43