Looking forward to community engagement and social aspect the most - people buffing each other, helping with combat and quests, strangers becoming acquaintances, and acquaintances becoming friends.
Since there's (hopefully) no server hopping, no sharding, no cross-realm groups, no character changes - your reptutation is tied to your name, and always follows you. And considering Classic's design encourages organic teamwork, if you're being helpful towards other players, they'll most certainly return the kindness.
Dungeons, raids, quests, atmosphere... that's all still great to me, but in Classic, it was the community that created most memorable moments.
One unbreakable shield against the coming darkness
One last blade forged in defiance of fate
Let them be my legacy to the galaxy I conquered
And my final gift to the species I failed