One thing I've learned is that "charisma" matters more than anything else.
I don't think this is a great explanation. I think charisma and likeability and whatever else are not that important, and really this election helps to show that.
The way I've been thinking about this for a while now is that we have this very tribal politics. People really only think about politics anymore as a zero-sum competition between two sides, or maybe three including The Establishment (but often this is rolled into the other side). And people feel like what we need to do is simple and obvious. Their problems are pretty easy to fix if only someone who actually wanted to fix them could get elected - if only we could elect someone from the right side. So when judging candidates people are looking above all else at that - the big question is just what side a politician is on. People are looking for markers of tribal affiliation.
Clinton's weaknesses here are obvious. But the really important dynamic is what's often glossed as racism (and I'm not arguing that it isn't racism). Because lots of non-college whites understand themselves to be on a different side from minorities, minority outreach of any sort is going to turn them off. They don't need to understand themselves as having anything against minorities, and they don't need to be hateful or discriminatory or anything like that - they just need to have some not-necessarily-conscious sense that their political interests are opposed. So if you're on their side you're not on our side.
Trump's strength with white voters derives from what you could call "vice signalling". He's strategically non-PC in order to demonstrate that he must be on the side of white voters. Nobody who's aligned with minorities could talk that way. And The Establishment hates him too, so clearly he's not with them. So he must be with white people, since that's the only other option. There's not really room here for a notion of a politician as just a jackass. Many of them are genuinely offended by some of the things he says, but those same things that offend them also demonstrate that he's aligned with them and is likely to serve their interests in office. None of the specifics matter and many are happy to excuse various insane things he says because, again, any idiot could solve our problems as long as they actually tried to. And he's got at least a few ideas that strike them as exactly the simple and obvious solutions we need, like tearing up all the trade deals or whatever.
I think this goes a long way towards explaining Sanders' popularity too. I mean, he was certainly not charismatic in any normal sense of the word. The appeal was and is pretty clearly about how he's on our side and would be a leader who would
finally take the obvious steps that need to be taken and which have been avoided so far because everyone in power is corrupt. And he demonstrated this to Democratic primary voters in a variety of ways. Notably, though, he didn't really connect with minorities. He didn't come up with a good hard-to-fake signal that he was really on their side, and since he got a late start he was always going to be viewed suspiciously. I suspect this very failure to connect, in itself and totally independently of any policy he put on his website, would have helped him with white voters in the general, though obviously being an atheist Jew socialist was going to hurt.
Edit: Obama's big advantage was that he could carefully avoid any sort of racialized language. He could talk as if he was only trying to win white voters, but black voters would trust him anyway. White voters didn't really hear anything that suggested that he wasn't on their side. Jeremiah Wright and others were second-hand, and he denounced them. Like, even if you think it's all racism, racists aren't stupid and they're a lot more likely to vote for a black candidate who won't push pro-black policy than they are for a white candidate who will.