I really loved the zone designs, and how alien (“otherworldly”) Shadowlands felt — I even made a few posts back in the day pointing out that Outland/Draenor shared assets so you could recognize that Draenor would be the Outland, and that the Outland was Draenor. I likewise pointed out that Kalimdor, Eastern Kingdoms, Northrend, Pandaria, Broken Isles, Kul Tiras, and Zandalar all shared assets so they were recognizable as being on Azeroth. Shadowlands, by contrast, had unique assets, design, and mobs. You cannot mistake Shadowlands for Draenor/Outland and Azeroth, and I loved it.
I also enjoyed the class design in SL, compared to WoD/Legion (although DF was a definite step-up).
I enjoyed the fresh new lore, although I’ve always been in the minority.
I enjoyed Torghast, too; I liked challenging myself.
However…
SL was very, very, VERY grindy.
It was horrendously grindy.
THAT is why, at the end of the day, I rate SL as a mid-tier expansion — better than WoD by a considerable margin, and far superior to Legion, but inferior to all the other expansions.
I burned out on the grinding by the time Zeneth Mortis dropped, so I stopped playing there by the time I hit revered with the Enlightened — I was sick of the grinding. (And I can handle grinds! I have the 100 Exalted Reputations achievement. Look at my achievements for various grindy things.) I spent the reminder of the SL expansion working on Torghast and older expansions.
TL;DR? It was the totally absurd grinds that really hurt SL.