Gameplay
As this is not a talent we have been playing a whole lot before, this means the playstyle changes slightly. That being said, Dimensional Rift doesn’t actually interact with a whole lot of our kit, meaning we pretty much just get a new damage source, that can be used on the move. In season 2 you had to consider when to use Channel Demonfire to proc the damage increase from the set bonus, but in season 3 we just get a whole lot more Dimensional Rift casts, and these have a chance to proc a flame rift that deals more damage.
Strength
Looking at raiding, this set bonus is fairly strong, for a few reasons. We get more damage on the move, as Dimensional Rift is an instant cast. We can sit on a stack of this at all times, and then use it right when we have to move. Previously we only had Conflagrate to cast on the move and if you didn’t you would just have to accept the downtime in dps. You could also consider saving up multiple stacks of Dimensional Rift, to then deal more on-demand dps onto a priority target. Although with how the 2-set works, you could risk losing out on Dimensional Rift casts so this may not be worth it in all situations.
For dungeons and Mythic+, this set bonus is not what you were hoping for. In Mythic+ Destruction has largely been playing a setup that included a lot of Rain of Fire casts, proccing Ritual of Ruin and Avatar of Destruction. This setup benefits greatly from Chaos Incarnate, which is the alternative talent in the same note as Dimensional Rift. This means, in order to pick up Dimensional Rift you would be losing out on Chaos Incarnate. Some Warlocks have even been discussing if this would end up being worth it, as the only Dimensional Rift that deals any AoE damage is the fire portal from our 4-set.
It’s going to be interesting to see what version of this set bonus goes live because, in its current state, it looks like a set bonus that only really benefits single target and light cleave. And if this set bonus doesn’t change, what that will mean for the Mythic+ talent setup for Destruction. If this version goes live, it’s very unlikely Destruction will be a good contender for a Mythic+ spot.