Personally, I think what f**k up Clinton last time was Clinton. Her herself, as a person. Her entitled attitude, and the sense of inevitability she seemed to exude. She was asinine and presumptuous where Obama was respectful, and for a politician, humble. It was the contrast between smarmy Clinton and unassuming Obama that did it for her.
I think its a mistake to project too much rationality onto the voting public. This is a "gut" decision for a lot of people...who, lets face it, aren't bothering to educate themselves on the issues, and are prone to internalize soundbites and such.
But now we have the contrast between Sanders and Clinton. Sure, Sander's makes a hell of a lot more sense, is clearly more idealistic and impassioned about left wing issues...but he comes off as frazzled and old and presents himself not so well. His New York accent does not ingratiate him with middle America. He's not photo or telegenic. He has nowhere near the name or brand recognition. And he doesn't shy away from the "S" word. Which isn't going to bother people who actually know what its about...but to most people its just a bright neon red hammer and sickle flashing in their brains.
This isn't about the issues anywhere near as much as its about the people standing up on that stage, in front of those cameras. Its not so much what they say as how they say it. Clinton is just better at this shit than Sanders is. If you slur out the words, sanders can almost remind one of Ron Paul when he gets up there and starts rattling on about something. He's not coming off as "presidential" as Clinton is. Not that presidential is a good thing...but its something that the voting public will recognize and respond to. This is more like people voting for someone on a reality TV show they like better. Rather than voting on people who's ideas they like better.
Clinton can still sink herself, if she turns up the asinine, cocky, entitled thing again.