It depends on what kind of content you enjoy.
PvE players had the same ol’ same ol’ with the raids being meticulously crafted.
PvP players didn’t really get anything new though and on top of that, there was the usual rough patch in the beginning like Feral Druids taking out 80 % of your health with one Ferocious Bite and Frost Death Knights being similarly dumb.
There was also some new progression system for PvP.
You had to deal with Ashran (aptly named Trashran, since you fought more PvE Trash than PvP encounters). This is also the expansion that introduced RNG drops in BGs. You could still select certain pieces through an NPC with the honor currency though.
But regardless of whether you like PvP or PvE, everyone had to deal with the mobile game inspired Garrison. Time Gating the likes of which we hadn’t seen before. Some of the gating was reasonable, but almost every option came with a cooldown incurring, which very quickly became frustrating.
Furthermore, this largely replaced people’s tendency of hanging out in their capital city, because you had most things you needed in your Garrison. Keep in mind that it’s phased, because they had to provide the illusion that your Garrison was unique and owned by you (even though they mostly looked the same, with few differences).
Apart from this, most players went to do their dailies which were largely remade from previous iterations. Not quite world quests, but the whole “Complete bland, undefined objectives within this proximity to complete the quest”, originated within WoD.
So once you had painstakingly sat through your garrison for the first few weeks, most players began to realize, that this was the expansion. That didn’t sit well with a lot of people.
Then a long time after, we got 6.1, the infamous “S.E.L.F.I.E” Patch, which was advertising a Toy that tied in with external Twitter integration. New additions to the monthly event of Darkmoon Faire, displayed as major features. I’ll let you guess why this didn’t go down well.
There are other things such as the lore being a time-travel/alternate dimension plotline, which is almost always something writers resort to when they’re out of ideas and it frankly showed.
Class-pruning also hit some classes hard (rest in peace destruction warlock and Windwalker Monks).
It’s actually a shame that it had so many rough parts, because it had some genuinely amazing stuff in it too. Not even talking about the raids, I wouldn’t know, but many classes had some really cool specs to play. My all-time favorite iteration of Retribution was in WoD. Same with Demonology Warlock.
The story of the expansion, while it was in fact a lazy timetravel/alternate dimension approach, actually had some really cool moments and characters.
The lore surrounding a non-broken Draenor was also cool.
But there’s no doubt that the expansion was lacking in content.