The idea of RE.
My mom was very frugal and money-conscious, so I learned a lot of the "right" things. But she also loves her work and never intends to quit (when she turned 60, she opened her own consulting company on the side; now at 70, she is retiring from teaching because she makes 3x her salary consulting. And she still lives on $30K/yr.). So the refrain I remember from my whole life was "follow your passion" -- the whole "find a job you love so much you'd do it for free, and it won't feel like 'work'" thing.
The problem is that I never did. I have had good jobs and bad jobs, but nothing I'd ever consider doing if they didn't pay me (well). So for me, I wish I had known the shockingly simple math 25 years ago, because I *thought* I was doing well saving 20%+/-. That knowledge would also have allowed me to have very different conversations with DH over the years. I only started pushing for RE within the last year or so, as I seem to be terminally bored at my job. But the problem is that even though we are FI, jumping ship now would require a significant lifestyle change from what we have been planning over the last 20 years, and he just isn't willing to buy his/our freedom at the cost of the vision of the future he has been working toward for so long. But if I had known about RE 20 years ago, I'd have had decades to work that into our shared vision, and we could be there now.