I mean, I think the DH example given by MagdalenaDK that Bacon quoted hits the nail on the heads to anyone who's played DH for some time, and it's a design that's been ported over to many other classes.
Whatever we think about how metamorphosis SHOULD belong to DH, the fact is that they removed a much more nuanced metamorphosis mechanic from demonology warlocks to give DH's a completely shallow version that basically just gives extra damage to the same 2 skills DH's already use. So much so that in Legion, demonblades led as a talent so much because it skipped a major downfall of metamorphosis which was GCD's spent on demon's bite, which didn't change with metamorphosis so demon blades was significantly better since the resource to be spent during metamorphosis didn't clash with the mechanic of metamorphosis. The only reason that is no longer the case is because Blizzard had the brilliant (/s) idea of sticking Immolation aura, an aoe talent, in a row of purely single target traits. Hey, who knew, that when given the choice of a talent that boosts your aoe vs. a ST talent, most people would pick the aoe talent given the importance of M+ and current raid encounterdesigns -- especially if said aoe talent is also fairly close in ST to the purely ST options.