It's good to discuss the pluses and minuses of an approach, we encourage this in our architecture section of the interview loop and during the initial screen when discussing line items in the resumes.
It's having a general negative attitude... bad mouthing past employers, being negative to a tech stack (and literally not couching it with any reasoning what-soever when we try to dig in), being negative to past arch decisions by co-workers without providing what are better solutions, things like that. Sometimes it's implied, sometimes it's more direct, but it there is a distinct lack of social intelligence that seems to happen more w/ older candidates that just suggests someone is burnt out. Wisdom should bring skill, not attitude.
Forest from the trees here. You pay someone to bring value to the team. That is not valuable to anyone. We're not running charities.