For me, it was the community and a sense of accomplishment when you progressed through the game.
About the community. It was certainly more a community back then. You would get to know people. Often times, if you needed help, others were more likely to help. We actually talked to other players. I’m not saying LFR is bad, because it’s not. But in I wanted to run a dungeon, we would put together a team. Someone would say “Hey, I know this tank” or “I can swap to my healer.” You got to know peoples names, at least their toon names. You made friends. In LFR now, it’s a quick group, kill stuff, leave. No one typically talks.
Our guild would get on Vent, and we would raid Molten Core, or whatever. Someone would be blasting music in the background. But it wasn’t loud enough to overpower anyone speaking.
As for Accomplishments, When you got your mount at level 40, it was great. I remember being in Desolace without a mount. And back then, Desolace was just that. Desolate. It was nothing more than just a grey area with mobs.
Then, when you went to Sunken Temple. and did your class quest. That was a great feeling. As a Warlock, mine was the Soul Harvester, which at the time was the only scythe in the game.
Then, doing the Walock Dreadstead quest. That one was a pain, because no one wanted to do that side of Dire Maul. And the quest itself once you killed Immolthar, and started the actual quest. All those mobs that spawned. Having to restart the ritual items. It truly was epic.
I remember, after getting my Dreadstead, sitting on top of the bank in Orgrimmar, just taking in the view. Taking screen shots. Showing off my new ride.