I have only had some previous experience with Project Euler, which I liked for the loop of "try to bruteforce it -> doesn't work -> analyze the problem, exploit patterns, take shortcuts". (I hit a skill ceiling after 166 problems solved.)

Advent of Code has this mass hysteria feel about it (in a good sense), probably fueled by the scarcity principle / looking forward to it as December comes closer. In my programming circles, a bunch of people share frustration and joy over the problems, compete in private leaderboards; there are people streaming these problems, YouTubers speedrunning them or solving them in crazy languages like Excel or Factorio... it's a community thing, I think.

If I wanted to start doing something like LeetCode, it feels like I'd be alone in there, though that's likely false and there probably are Discords and forums dedicated to it. But somehow it doesn't have the same appeal as AoC.