wc3 Arthas dumbed down to a meme villain in wotlk, with only good execution of him in the death knight starting area and ICC.
Also, they wasted him in 1 expansion, and lets be honest it was the best wow villain up to date, they are more powerful ones, but they are not as developed lorewise, or lets be frank cool.
then another one: introduction of machines in bgs, i remember nobody liked it, increasing wait times in que for this mess.
Mal'Ganis revenge arc - wasted.
Yogg Saron basicly wasted, too, although in a rather good raid boss mechanic wise, but in lore he was wasted for a raid with no connection to arthas really explained.
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BfA over Legion? Are you an artist?