I think the line of argument Atrios is complaining about here is a natural response to differentiating between two people who really aren't that much different. Yes, there are real (and I think significant) differences between Hillary and Bernie, but they are really quite insignificant compared with the differences between them and any Republican or them and Trump.
I think Hillary would be better at the logistics of the presidency and Bernie would be better at mobilizing [Democratic] base voters, which could move the legislative window of the possible a bit more liberal. Obviously I am more aligned with Bernie in terms of policy, particularly foreign, but I don't feel strongly enough to try too hard to persuade others...and I have strong opinions. I think the people trying to persuade are picking nits and most people don't care, so it becomes frustrating and then they lash out.
There is a caveat to not caring and it is the general election. I think lots of Democrats view a Trump nomination as gifting Dems the White House, and my concern is that Dem and Dem sympathizing voters will become too complacent. In a Trump vs. Clinton matchup, between the (irrational but widespread) loathing of all things Clinton (and, yes, Hillary in particular, and in part because she's a woman) and lots of complacency because "Oh, Trump can't possibly win" mentality I can see him winning that election.
The presidency of the United States should not be a joke.