There are elements of Shadowlands that I enjoy (in terms of aesthetics and cosmetics, it’s my favorite zone ever). But for me, what bothered me most was how it existed in a sort of time/lore vacuum.
Roleplayers like myself don’t typically see our characters as “the main character” who is actually doing the campaign quests; we like to see ourselves as little guys doing side quests in the background and helping out as part of a larger army. Shadowlands was very “main character” focused, to the extent that most normal people in Azeroth probably wouldn’t really know about what was going on there. This made it incredibly difficult to engage with as a roleplayer. It essentially excluded us from the whole expansion.
I also just found it very hard to care about any of the zones or their characters. Because not only were they entirely new and unfamiliar—but also, we knew we’d probably never see most of them again once the expansion was over. They sort of just exist in this bubble that is entirely disconnected from all other plot in normal WoW. It’s pretty hard to get invested in something that you know isn’t going to matter long term.
I know there are gameplay reasons why people hated SL, but I personally didn’t do anything beyond extremely casual engagement with the game during that expansion. Quite frankly I didn’t feel motivated enough to try at all. It was the only expansion where I unsubbed from the game. It wasn’t so much that I “hated” the lore (though I admit I didn’t care for Zovaal, or for Sylvanas’ character assassination), but more so that I just found it inaccessible to roleplayers and ultimately irrelevant in the long term to the world of warcraft I fell in love with nearly two decades ago.