Unfortunately, Power Word: Barrier has yet to make an appearance on the PTR, and it turns out that we may not see it at all. It's still a possibility, but that's the extent of it. I can sort of see why they would hold off on it, or even change their mind about it. Discipline has gone through a lot of changes in patch 3.1, most of it a really big numbers game. Discipline is a great tree, and while most players might not 'get' the damage mitigation/prevention thing, it's a really powerful mechanic and probably one they need to be careful with.