What's interesting to me is that "bitter" vegetables from different families have different qualities of bitterness. I love Brussels sprouts and kale and all the other crucifers, but the earthy bitterness of Swiss chard or winter spinach needs a LOT of lemon.

That earthy part, just literally tastes like dirt to me, I don't find spinach or chard bitter at all. There are a bunch of different unrelated compounds that all give some sensation of bitterness.