One of the best Pro's about Priests was that they had Fear Ward, and then they allowed any race to have it (although they increased the recast). The huge flaw was that it is a Holy Spell and was not able to be cast in Shadow Form, making it a huge mana drain to switch forms - and taking up valuable time in a heated battle.
When I toggled around with Shadow Priests doing theory crafting, some PTR tests and actually leveling up some on my own this was a huge drawback. Now though from a confirmed Blue source it will be castable in Shadow Form.
There is a ton of other awesome changes that have been announced for the 2.4 patch and more details are coming but as for the Dual-Boxing community this is a huge boon for anyone that has or is considering to roll Priests.