Revealed meaninglessness. The show was thick with the feeling that mysteries were being uncovered and resolved. The final season killed the suspension of disbelief. The producers and writers had no real story to tell, just the ability to make us feel they were telling a story.
80% of Lost was the build up. Contrast that with another show with a huge reveal like Westworld season 1, but that's 80/20 the other direction, and it has high rewatching value.