Strictly speaking arena, mop was actually pretty fun/good.
Sure there were a few things that were broken here and there, but there were in every expansion. In wrath, you had Death Knights running around 2 shotting people with icy touch, mutilate crtting for half your health bar, druids who were not viable healers, and thats all just in the first season. Then you had prot specs running rampant later, and eventually orange weapons and full pve gear playing double healer warrior.
Dont get me wrong, I loved wrath, and look back on it very fondly, but it was far from perfect.
In mop, you had the same shitty start as wrath, with hunter stampede 1 shotting everything, and then you had this
later we saw rise to elemental shaman and the meatball spam. But the expansion did have its good bits for sure. The most noticeable thing is that tyrannical, grevious, and prideful (moreso the later two) comp diversity was high, similar to how it was in season 9 (arguably the best season of cata), and season 8 (probably the best season of wrath). When we look at other expansions, like wod and to a lesser extent legion, we dont really see a high diversity in team competition. There is always a "best" class this or that comp, and that class will outshine all others of its type (for the most part)