Great lore. Awesome fantasy. Historically complex (bfa notwhistanding...). Rewards investment. in mop and wod, amazing class design with three so-fun-its-unfair speccs. The strongest "watch them rot slowly" theme with affliction in arena. Class quests like green fire and classic epic mount. Its just too good. BFA broke my love of wow by ruining affliction, but you never forget a first love.
Edit:come to think of it, the mop (and less so wod) incarnation was also the epitome of agency and decision making. No class that I'm aware of had the level of decision making an affliction lock could have with soulburn. You could use it for instant triple dots, a finisher, but also tons of non dmg utility (insta pet summon, enhanced drain life, demonic circle port with speed boost...). It was a pvper's wet dream and it rocked in pve too.
The same goes for demon form in demonology. At almost any point you could shift to a 600% more armored form with a heroic-leap like warriors, but then you may not have it when you want to go for a kill. Nothing before or after in the history of wow class design compares, imo. And yeah chaos bolts rock.