Besides the concept of FIRE in general, of course:
-The idea that challenge and small discomforts are good. I often do the harder or more physically demanding thing intentionally now, and the challenge of it makes me enjoy things more than doing things the "easy" way. Walk/bike to get groceries, run home from work, train/bus instead of Uber, walk to hardware store and carry heavy things home. It makes things in everyday life feel more like a challenge and I enjoy that.
-Hedonic Adaptation--if you always have amazing things you'll keep needing more and more. And reducing the number of amazing things actually makes those things seems better. Eating super basic meals just for nutrition for breakfast/lunch often, and saving the fun, tasty things for dinner. Making overseas vacations more rare, rarely going out to eat, finding fun near our house instead of needing to do lots of travel. It saves money and makes those special things feel more special.
-Frugality as environmentalism. I quite enjoy this perspective from MMM and it moved me further into this way of thinking. I started biking for fun, then taking the bus to work, then eventually sold my car and moved somewhere with better transit. Come to think of it, I wouldn't have been in this mindset at all without this blog.