It's a lot of things honestly. The most obvious is the ability to completely get lost in a musical space. My belief is that the best gear gets out of the way and you forget you have the headphones on. I have a setup that can more or less do this. This is why I stopped (for now at least) climbing deeper into this money-pit hobby. But I do still enjoy nerding out in forums like these and also learning as much as I can about what technologies lend to the best possible acoustic experience.