I noticed on the WOTLK servers that Demonology warlocks get an ability called Shadow Cleave, which is basically Cleave but shadow damage.

It's just weird, because it's a melee replacer, like heroic strike, cleave and rune strike are. But, it pauses the melee swing timer by itself. Imagine if heroic strike paused the melee swing timer like hard cast Slam does.

Additionally all spell casting pauses melee swing timer, that's fine. But It further puts it out of place on Demonology, as you're always casting. The only way to access shadow cleave is with two instant casts back to back or during movement.

I'd like to suggest that either shadowcleave becomes an off-GCD instant cast, or during Metamorphosis the melee swing timer isn't arrested by spell casting.