If Warren somehow managed to win both the primaries and a general election campaign, why wouldn't a President Warren feel just as free to criticize banks and our rigged economic system that favors banksters and the wealthy as she does as a Senator? She cares about issues that affect ordinary Americans, not her own re-election campaign. She's been the most honest and outspoken member of Congress in decades. Because she's not a career politician, she's not worried that pissing off wealthy donors will hurt her chances for being re-elected, she is just an outspoken defender of the middle class. And unlike most Dems (including Obama and Hillary), she actually means what she says and has real ideas on how to fix the problems that are squeezing the middle class out of existence and pushing all the money to the big banks and the top 0.01%.
ShadowOfTheWave is right that Warren was very much against the idea of becoming a U.S. Senator....yet look at where she is now. The activist base could convince her to run for president. An unwilling president who only reluctantly accepted the idea of running for the office due to popular acclamation would be a refreshing change from the long line of egomaniacs who have long occupied the White House and have run this country into the ground.
Elizabeth Warren is the most sincere person to grace the U.S. Senate with her presence right now and as president would definitely be the most sincere president probably since George Washington who as we all know couldn't tell a lie since his days chopping down cherry trees.