Disclaimer: I got a return to WoW offer during late WoD, after the last patch to it, and have been playing since then.
This means I didn't suffer the slow patching and content drought of WoD for too long. I think it is better in quite many ways than Legion, but Legion added lots of content rapidly. But now we might have to wait for 6 months to nearly a year for Battle for Azeroth, and I found Tanaan Jungle > Argus, by far.
What WoD did better: Story execution, questing. In general I like the demon invasion of Legion better though. The Garrison and related stuff was more fun to build up than the Order Hall, but I like the champion/expendable helper troops and fewer missions to click of the Order Halls much more. What I don't understand is that they didn't remove gold missions from the Order Hall, they now fill my pocket just like the garrison did. I know how much cash vets made from garrison gold and resources. Building up the fleet was quite fun, I somehow would love to have a fleet/ship order hall thingie in BfA!
I liked the shorter WoD dungeons much more than the longish ones of Legion. I do a weekly 15-16 Mythic+ dungeon on my main and otherwise avoid them like the plague! Especially disappointing: Karazhan. I farmed this raid to death, making a raid a dungeon, just too longish, even after splitting it in two parts.
I like the Artifact Weapon stuff and upgrading it. I would have liked to collect "power" that gets applied evenly to all artifact weapons, so that switching specs is possible and not so much grinding focus on one spec is required. Basically locking players into one spec was a bad idea. The armor infusion system of BfA seems boring in comparison, collecting artifact skins was also fun.
The legendary system was rather bad, too random, too many legendaries, made them trashy, the constant upgrading of lots of legendaries became quickly quite annoying, too.
I like the World Quest system and the emissary chests for three/four world quests very day, able to finish the objective over 3 days. That's quite nice, a daily quest with a bit more time than a day! The drawback of the world quest system is that the normal quests in Legion were few, short and mostly boring. Or they are like the really great Nightborne story arc, wonderful once, a total chore for alts.
Raid-wise I liked the Nighthold, great raid. I never experienced the early WoD raids before they got trivial, but liked Hellfire Citadel a lot. Antorus is weak, Tomb of Sargeras was good, but not Nighthold or Hellfire Citadel level. The filler raid Trial of Valor: Did it only few times, interesting encounters but I just don't like these filler in between major raids stuff at all.
Class design: Not happy with many class changes, I am super happy that Affliction got this funny soul explosion stuff though. Demonology is horribly boring. The Survival spec for Hunters can be quite powerful, but I find it has a really bad flow and doesn't feel good. I heard Unholy DK is good but tbh I only played Frost and Blood this expansion. My Pally works out quite nicely, barely remember how it was before. Beast Mastery is a bit too simplistic by now, don't like Marksmanship. Demon Hunter, hum, already have too many melee, it's not bad but not a class I will play much in BfA. Feral Druid got patched nicely close to the end of Legion patching, bear tanks are still too strong IMO.
PvP: I don't PvP much, no comment.
What I really liked about Legion was the fast new content delivery. I could live with slower updates, really! What surprised me was how many ideas and effort went into the Broken Shore, only to get supplanted by Argus stuff little later.
Verdict: WoD was not nearly as bad for me as others found it, I quite liked it, enough to resub and play Legion. I always had a soft spot for Draenor, the alternate timeline Draenor was fun for me to explore. The Broken Isles were prettier, everything densely packed together, but it didn't have this "world" feeling. A bit too small. I liked the Artifact weapon and legendary stuff in general, it had some huge flaws as well, unfortunately.
I would rate Legion a bit higher for more things fresh and fun to do late game, but as for endgame zones and raids, I liked Tanaan Jungle and the Hellfire Citadel more.
Edit: What I hate about Argus, it's so smallish and so much mini bossmob shooting gallery. Maybe fewer larger ones instead of oh so many mini bosses.