1) Bonus quests and apexis dailies are basically switched around. Bonus quests in WoD were there for leveling and had objectives, apexis dailies were for end game and were simply 100% bars. In Legion, bonus quests are world quests and are for endgame. The 100% bar quests are for leveling and serve as a bonus to complement quests you are already doing in that area.
2) No flying in WoD was pointless because there was nothing to do and they missed the opportunity to show the community the advantages of not being able to fly. In Legion there are things that wouldn't work with flying, like the world quest in Azsuna, where you have to use the bubbles to get to the top of the building. Or the grappling hook in Stormheim. Also, they put things in like flightpoints that make sense. Like when you go to the Warden island in Azsuna, and you wanna do a world quest, it's an elite area, so you will have a hard time getting through the first time, but there's a flight point on top, that, if you're there for the first time, you can get, and then every time you return to the area, you can fly to the top and drop down to where you need to go, avoiding having to go through the whole area EVERY time, which feels like a great reward and like you unlocked something.
3) Tanaan Jungle as a max level zone felt uninspired, with barely any story taking place, only having 100% objectives and a few campaign quests. Suramar is an awesome zone with a great story, great characters and a great environment. The Nightborne and Nightfallen are awesome. Characters you can empathize with, even the Nightborne, because a lot of them are actually against the Legion, but dare not to speak out, out of fear of being thrown out of the city. Suramar city looks amazing. It's gorgeous. I wanna live there. The whole zone looks stunning.
4) The old apexis dailies (100% completion bar) are utilized in a better way. You don't have to kill as many things as you did in WoD, which, when you have to kill dozens of mobs and the bar only ever advances a tiny bit, is really tiresome and demotivating. In Legion, it's a lot faster, and since you get them in areas where you are leveling up, you are just doing them on the side, while also doing real quests. Also, doing them at max level works well with other people leveling up, cause you can kill the same mob some low level character is killing for his low level quest, and you can still both get credit for it, for whatever you are doing in that area, be it quests to level up, or world quests at end game. It encourages helping each other, especially with the new tag system.
5) From what I can tell professions for the most part don't have these awful cooldowns on things you can create. And there are no profession buildings like the herb garden and the mine to devalue mats.
6) Daily quests always felt like more of a chore. Something you needed to do each day, because you could only do a certain amount each day, and you had to make sure you don't miss out on any. With world quests, it doesn't feel like that at all. There are always some world quests. They are on different timers. There is no "I have to do these quests until the reset happens real quick, and then the whole things starts again". It feels so open and free. You just log on, see what rewards interest you, and just do the quests you want to do. It's the best thing that happened to WoW in a long time.
That's as much as I can think of right now. I'm sure there's more stuff. Overall it just really feels like they've learned from their mistakes and took it as a chance to improve on things that went terribly wrong in WoD, even though they had good intentions at the time. I guess sometimes you just have to try something, risk something, and see how it goes, and WoD was simply a failed experiment.