Compassion is probably the one lessons I truly want to hammer home for my children (including the ones I only claim when they are at school with me). The idea is simple enough: consider the other person. Think about what that person might be feeling. Consider that there is history you are unaware of that might make a small thing more painful than it seems on the surface.
Around the middle school I teach in are several versions of the idea, hanging on posters outside various teachers' classrooms. In middle school, we have to fight the blurt factor. Kids this age have a thought and say it without considering the consequences or the effect on others. They often don't have ill intent at heart; they simply didn't THINK:
But what happens among the adults out there? The ones who value their own zinger of a joke over the heart of a human being, or who have simply never outgrown their adolescent narcissism? For me, I've started to call them on it. Bullying among adults is just as large a problem as it is among children. Larger, maybe, because the kids are more likely to learn and outgrow it. But bullies will keep bullying as long as they get away with it. So, when you see it, speak up! It's not as small as it sounds.