Well... I think the most fundamental one is simply that "My thoughts are not myself." 95% of people - i believe - simply attach to the thoughts that magically appear in our minds... and then they "believe" that those thoughts must be "who they are." In fact, having the ability to "choose" the thoughts we want to attach to is a much more fulfilling way to live. Left undirected, the mind will by default move to lazy, scared, distracted, divided, and disappointed. That is what the amygdala evolved for 300 million years to do. However, we can train our ability to choose our thoughts. THIS, I believe, is the most important idea that people either forget about, or were never taught in the first place.