For the record, I'm very much a Vanilla/TBC fanboy. I don't think the game was perfect back then, but I do think it was better and felt like an actual world. I think the game really started to change for the worse towards the end of Wrath and has never really recovered.
I played Cata until it the first patch was released before quitting. I only played MoP during a one week free trial before WoD released because try as I might, Pandas really ruined the image I had of WoW as a world of primarily high fantasy (yes, I'm aware you can play as a talking cow, but the narrative in Vanilla/TBC/Wrath was centered around typical high fantasy tropes like dragons, demons, and the undead. I relapsed for the first time when WoD was released. I played for maybe two weeks, but it was evident within hours of playing that the game had continued to change for the worse.
I've played Legion for the past week and have already stopped. I will admit that there is far more to do than there was when I played WoD, but the core issues that many people have claimed precipitated the game's decline still exist: LFD/LFR, a billion different teleports, AoE spamming dungeons, CRZ, class homogenization, transmog, the relative ease of getting gear etc. On top of that, the game feels so bloated with things like mounts, toys, and cosmetic things that nothing feels special anymore.
Legion's questing, while praised by much of the game's community, is as hollow as questing has ever been in WoW. You kill 10 mobs, collect 8 items, kill a named mob, and return to the quest giver. The only significant difference with Legion is that you can play through the zones in any order you want due to the scaling technology. I also don't like that hitting max level took around 12 hours. Accelerated leveling has only contributed to the problem of split raids, where in order to be as competitive as possible, guilds run multiple raids stacked with alts in order to feed gear to a few people for "main" raids. This didn't happen in Vanilla and TBC because of how frontloaded the game was. I think the world record 1-60 in Vanilla was just shy of 5 days. Most people who know what they're doing take about 7 or 8 days. The first time through, many people took even longer. It took me about 14 days to hit 60 for the first time. The leveling back then was as much part of the game as end game was. The main problem with every expansion has been that leveling content becomes largely irrelevant when you hit max level. Fortunately, Legion took a step in the right direction to correct this issue by scaling the difficulty of zones to the player's level and scattering world quests across every zone. The problem is that world quests themselves are not fundamentally revolution, they're just randomized daily quests.
While I enjoyed Legion more than the little time I put into WoD, it still has a lot of problems. At max level, the game rapidly became about increasing two of my character's numbers: item level and artifact power. I wasn't hunting for specific items from specific dungeons, but equipping anything that was an ilevel upgrade. I wasn't doing world quests with an end goal in mind, I was just doing them for more artifact power that became exponentially less useful. On top of that, I was still playing garrisons with class hall missions. I wasn't doing anything like farming Aldor Rep for substantive shoulder enchants or farming BRD for a Barman Shanker. I was just looking for a larger number.
A lot of the game systems introduced with Legion are inspired by Diablo 3. I know a lot of people think that's fine, since those systems were generally well received by the D3 community. The problem is that D3 does not have the staying power as other games. People play for a few days or weeks at the start of a season before stopping entirely. WoW has been like this for a few expansions now. People resub for a while to play through the new content and then quit until there's new content again. The thing is, WoW didn't use to be like that. Sure, there were always people who would quit for a time, but from Vanilla to the end of Wrath, the game was continuously growing and millions of people would maintain a continuous sub.
Anyone who missed Vanilla or TBC really missed out on something amazing. WoW was a cultural phenomenon. Now it's just another game.
I'm glad that I got to experience that, but I'm sad that the game has lost its soul.
My rankings:
TBC > Vanilla > Wrath > Cata = Legion > MoP > WoD
I have played TBC, Vanilla, and Wrath private servers on and off for the couple of years and have always enjoyed them more than retail WoW post-Cata. I know a lot of people like to claim people look back on earlier iterations of WoW with rose-colored glasses, but that's not fair. There are people who prefer Super Mario 64 to whatever the newest Mario game is now. There are still people who play old versions of Everquest. There are still people who play the original Doom. I'm not mad that there are people who love retail WoW. I'm mad that they get to play the WoW they love, but I'm stuck wishing I could pay Blizzard to let me play TBC.