I have 3 that I really like
Replaymaking - Obviously. It's like an art form hidden in the game. It even follows the same patterns as art history, with certain "movements" and styles being prevalent over the course of the game, like Realism, Destruction, and Skeets all had their time in the spotlight at different times. It's also cool how early replaymaking was impressive simply by being hard to move your tori in a sensible way, and now that everyone knows how to move, its the things you don't do that make your replays interesting. And since there's no tutorial or anything with replaymaking, everyone has a different style and process. It's really amazing how you can tell the difference between replaymakers just by looking at their movement. I fucking love replaymaking and the community around it so much
Clans - I love my clan to death. All the dudes there are really good friends, and I wouldn't have made them without the clan feature. Even if we never really do anything as a clan, the fact that we're all under the same banner makes everyone in it closer.
IRC - IRC channels are better than real friends
