There's been a lot of discussion on the topic of Kil'Jaeden's Cunning in 5.4 (most notably in this thread: http://us.battle.net/wow/en/forum/topic/9282218881
) but we've got some new info, and since that thread is growing rather large I think it's best to start up a new one.
We've been following all of your feedback very closely, and we're going to try a different approach to Kil'Jaeden's Cunning in 5.4.
In a future PTR build, Kil'Jaeden's Cunning will once again be a passive effect. Selecting it will allow the Warlock to cast Malefic Grasp, Shadow Bolt, and Incinerate while moving. There is no penalty, so you'll be able to move at full speed.
We still feel that the 5.3 version of Kil'Jaeden's Cunning isn't working out, but we don't want to "nerf fun" either. It's clear that many Warlocks simply enjoy the feel of being able to cast on the move, and we'd like to be able to preserve as much of that feeling as possible.
With this change, we can make the "penalty" of taking KJC that you don't have Mannoroth's Fury or Archimonde's Vengeance, which should result in more damage output in cases where movement is less critical (and we're working on some changes to Archimonde's Vengeance that should help with that). We're also still planning on buffing Fel Flame in a way that helps lower the impact to mobility caused by choosing one of those talents over Kil'Jaeden's Cunning, without nerfing it for PvP purposes.
Remember, we're still extremely early in the PTR process. There will be many more changes to the game before the patch goes live.