Before representing my calculation, let's preview some of the highlight in the recent PTS patches:
1. Using a Heavy Attack against an Off Balance enemy will now only consume the Off Balance debuff if the enemy is successfully stunned.
This new behavior also applies to the Power Lash ability. You can use Power Lash on an enemy until the Off Balance effect expires or is consumed.
2. Boss enemies can only be set Off Balance once every 15 to 20 seconds. After the first Off Balance debuff ends on a boss enemy, they will be immune to subsequent Off Balance debuffs until this immunity duration is over.
3. Flame Lash (Lava Whip morph): The Power Lash attack from this morph now has a 3 second cooldown.
Lava Whip: This ability and its morphs no longer have a cooldown when they can set the enemy target Off Balance. Striking a stunned or immobile enemy will always set them Off Balance.
Ok now to the math part!

First off, let's talk about PvE, or more specifically, the juicy stuff, trials. On this math test, we will count for how many Power Lash proc you can pull off in 20 seconds.
On live: Since Flame Lash can only set enemy off balance every 5 seconds, the rational result might be 4 Power Lash proc, but you're dead wrong, because that 5 seconds cooldown only apply to the boss to prevent you from setting it off balance, but not your allies. If you count on group play, the number will go up and up, which make magDK life a bless. Of course, if you talk about rotation, nobody in their right mind will just stand there and Flame Lash the whole way to victory, but this is just for math test.
On PTS: This is where things get ugly. If you try it alone on a target dummy, you'll see that the number does not change a lot, because target dummies are not counted as "bosses", so the 15-20 seconds cooldown does not apply to them. But in real trial, that 20 seconds cooldown is severe. And if you think you can proc Power Lash for infinite amount of time, then there's a 3 seconds cooldown on you to prevent that. Which means even if you change the target for example in vHOF, you still cannot proc Power Lash in that 3 seconds windows. So now on PTS, for every 20 seconds, you can only have 1 to 3 Power Lash, depends on what kind of off balance the boss is being affected by, it might be 10 secs, or even 2 secs (seriously, deep breath only provides 2 secs off balance). If we put this into a dps rotation, magDK might only be able to pull of 1 Power Lash every 20 seconds, which is crazy.

Secondly, we get to PvP. I'll be short because the point is already said previously. The cooldown of Power Lash is now on you, not your target. Which means, if you choose to switch target mid fight (which happens a lot in Cyro and BGs), you'll find yourself stuck with that 3 seconds windows without any Power Lash. But hey, at least it's only 3 seconds now, and not 5 seconds.

On the PvP site, this change might not be so critical, but omg with the PvE. MagDK is already a suffering class setup in PvE, with no group contribution, overall lower DPS than magSorc and magNB, and comes with painfully almost 0 sustain built in mechanic. What bothers me even more is what ZOS comment about this change, which is nothing actually. They put devs comment everywhere else, but this.
I'd love to hear what guys have to say about this. Am I right? Am I wrong? Cast your vote people!