The New York Times has the story:
Mrs. Clinton is expected to begin her campaign with a video message on social media, followed by a visit to important early-primary states next week, said two people briefed on her plans.
But for all the attention paid to how Mrs. Clinton would reveal her 2016 candidacy, little has been said about her reasons for mounting another presidential bid. Her campaign rollout is expected to provide voters, particularly users of Twitter, Facebook and other social media, a succinct rationale that she is best positioned to address an American electorate that has seen virtually stagnant wages for middle-income earners over the last 15 years.
Meanwhile, the remnants of New Republic caution that she has become a single point of failure for the party:
In Hillary Clinton’s case, though, there’s still a good argument that the Democratic Party could use a contested primary this cycle: not to toughen Clinton’s calluses, but to build some redundancy into the presidential campaign. It may even be the case that some of these Democrats with rattled nerves are less anxious about Clinton’s prowess against Republicans than about the fact that all of the party’s hopes now rest on her shoulders. Her campaign has become a single point of failure for Democratic politics. If she wins in 2016, she won’t ride into office with big congressional supermajorities poised to pass progressive legislation. But if she loses, it will be absolutely devastating for liberalism.
If you’re faithful to the odds, then most of this anxiety is misplaced. Clinton may have slipped in the polls by virtue of an email scandal and her return to the partisan trenches more generally. But she's still more popular and better known than all of the Republicans she might face in the general, her name evokes economic prosperity, rather than global financial calamity, the economy is growing right now, and Democrats enjoy structural advantages in presidential elections, generally.
If nobody serious challenges Hillary Clinton, nobody can be her understudy. In the near term that isn’t a problem, but if doubts about her inevitability develop late in the year or early next, the placid silence in the Democratic field will grow eerie.
We're still a long way from the 2016 election. Clinton may be the best we have, or she may just be the best one running. But looking at the other guys, I can't help but think we're still going to win.