While many (most?) mysteries remain unsolved, Lost's series finale "The End" was, nonetheless, satisfying because it didn't purport to have all the answers. Instead, viewers were comforted with just one: If you live together, you won't die alone.
Maybe I loved the finale so much because it validated one of my theories—giving me a Tracy Flick "ME!" moment—a theory I proposed early on in the season: Bardos, aka, death cycles. (I'd thought that the Island was what the Losties had dreamed up for their bardo phase, when it was actually the sideways universe that was the dream.) But mostly, I loved the finale for its emotional aspects, which could've come off as schmaltzy, but didn't. I usually go for the mythology and geek shit, but I couldn't help but feel incredibly emotionally tied to the show's characters, and I think the writers did a great job in recognizing how important to fans these ties would be, while also not robbing us of the most fun aspect of the show—coming up with our own theories about what just happened.