The combination of the members and staff. Although most people think that, it's... true. PC is like the world, in a way, and we all need to work together as a team in order to live our daily lives (though in PC's case, to make our daily posts on-topic and not breaking the rules). Yeah idk, something about those factors combined.
Other than that, probably emblems! Really, I love emblems for some reason. XD Oh, and profile customisation. Aaaand... the modcp. Well, just the report and ip searching parts, at least. Either way those are my favourite assets of PC~