I'll try to give a very serious take on this, since I somewhat fit the mold:
Bernie Sanders does not have the same economic worldview as many Democrats. He believes in an old-school labor movement. The problem is increasingly few Democrats have any connection to organized labor whatsoever, and so that’s why he doesn’t connect as well with many Democrats on that front. And on top of that, on most social issues, Sanders is basically an agnostic. I don’t really fault him for that, but the more activist liberal doesn’t really see him as someone who will really fight for them on abortion, guns, etc. Identity wise, he's a stereotypical angry old (white) man. To some people that's part of his charm, to others it's why they can't stand him.
But I suspect this doesn’t really answer your question. The more potent issue is that so-called Berniecrats can be straight up childish and juvenile. Now, jfern and Landslide Lyndon are on the very polar ends of the spectrum, but frankly there are far more people like jfern on the internet. And those people tend to be younger, more digitally savvy, but also probably just as dogmatic about politics. I suspect Lyndon spends a lot of time on Twitter, so he’s probably witnessed the behavior of Rose Emoji Twitter and not liked what he’s seen. Chapo Trap House, Cumtown, Jacobin, etc. just have such limited appeal to people above the age of like 25. And that’s probably where a lot of the resentment comes from.
Now personally, I also have a problem with the organizational apparatus surrounding Bernie. I don’t think I need to explain why I think Justice Democrats is an idiotic organization, but Our Revolution and DSA are also filled with some wildly ridiculous people, and they bring all the stupidity that comes with youth politics.
Plus I guess I resent that Berniecrats buy into the idea that Sanders’ platform is somehow broadly popular with Americans. That’s probably why I’m so averse to primarying Generic Democrat for not supporting Medicare For All or whatever, since I don’t believe it is actually that popular. Most of the country is not nearly as ideological as Bernie and his supporters, so I don't agree that people are clamoring for single-payer. It’s probably a conflation of Bernie’s popularity in general, which I credit more to his personality. And that’s the easiest part for the Right to take down over the course of a presidential election.
So to answer the OP clearly, the fear is that these Berniecrats will forcefully take control of the Democratic Party, and turn it into something amateur, far less competent, and probably less electorally viable.
I disagree with a lot of it, but that doesn't necessarily detract from the post quality.
