Community
Everytime you partied up with friends and the OG way of searching for raid members through dedicated raid websites. Doing them for hours on end and needing the wipe because either someone fudged up a step or didn't do it at all. To someone afking on you half way through.
Getting on to the weekly strikes to get new unique loot from vendors and saving up all those strange coins to meet up with Xur every Friday. Everything had purpose to it. It had replayability that you'd never get sick of because it was always rotated in a way that felt new every week.