Basically what Nixon had going for him in 1968. She's the pragmatic moderate, the elder statesman, the one with seasoned leadership, people having buyers' remorse over Obama, disaffection with both major parties, far and away she's the most qualified candidate on both sides of the aisle, and she's far more seasoned and confident than she was 8 years ago. She's not the same person that came in third in Iowa almost 7 years ago, just as Nixon wasn't the same guy that got caught on tv sweating like a fiend in the debates with JFK. Most importantly, they're clearing the field for her in her own party, and there's no significant opposition candidate from the other party. The only potential Republican candidate with her degree of experience would be Jon Huntsman, and there's almost no way he can win the GOP nomination in 2016. Even one of the popular governors like Kasich or Martinez doesn't have the foreign policy credentials to challenge her; Huntsman is the only one who can even compare as having both executive and foreign policy experience.
I like a lot of potential candidates in both parties including Clinton but I feel like there's something almost inevitable about her winning, and unless you have some sort of unforseen October Surprise in the weeks before election day, she'll be elected the next president by a comfortable margin. Warren is the only one who'd even have a shot at stealing the nomination from her, but she's said over and over she doesn't want to run. Sure you'll have Sanders and probably O'Malley and Schweitzer as "also-rans" but they're just looking for cabinet positions, or in Sanders' case to move her to the left in the primary (after which she'll pivot to the center again for the general)
She's also a about as likable as Nixon, although that's obviously a major weakness
