I’m surprised no one mentioned this, but for me it’s the way they use music to convey certain ideas and paint a picture of specific situations. The way they use the album format with a specific concept each time, and they use that to tell a story. It’s really unique, so nothing feels out of place. It sounds like the albums have a mind of their own and they couldn’t exist in any other form. So the sounds and music is only 50% of the formula. But they probably spend as much time working on ideas and concepts for an album, and I’m pretty sure that’s one of the main reason they don’t release as much, because the concept needs time to live on its own.