We're not talking things like a pay cut, or a change in benefits, or a bad boss/co-worker that you can't work with.
We're talking about things that the employer does that makes you say you can't work here anymore, out of principle.
I was thinking about this when I was reading how the San Francisco school district was going to lower standards to the point that you can pass with a score of 21% and that students would be allowed to turn in assignments late and retake exams multiple times (#992094)
I think I would quit if it came to that. I might stick around a few months to see if the policy gets overturned, but if it stayed I would quit.