I haven't played Shadow seriously since BC so I can't speak as to the gameplay implications, but I really like the concept of the shadow orbs. Hope to see more info about this soon!
The shadow orbs mechanic brings up a couple interesting questions. Mainly, I wonder what effect increasing Mastery at upper levels will do. Does greater Mastery increase the frequency of gaining an orb? does it increase the potency of an indiviual orb? What kind of internal cooldown can we expect on proccing orbs? will orbs become a kind of secondary resource if spells like mind blast can consume orbs for added damage?
In terms of adding value to the shadow rotation without fundamentally changing how it works, I think it's pretty cool. I think Mind Spike will be situational, but I can understand why they want to add it.
Assuming Leap of Faith is a Holy spell, there's a good chance it won't affect Shadow at all, but it would be a neat tool to give to a DPS class. And Inner Will... will only affect Devouring Plague with it's mana reduction component.
It'll be interesting to play with the idea of having Inner Fire up when doing the DPS rotation, but then switching to Inner Will when going on the run. The removal of the limited charges will make that very fun to play with.
Overall, the new additions to the toolkit are interesting. I think Shamans might have won out in terms of getting cooler toys to play with, but as has been stated, Priests have a pretty big spellbook as it is.
My translation of this preview: "Priests are pretty good as they stand, but we can tweak them to make them better. Here are a couple toys that will help with that."
Definitely interesting in hearing more, and definitely eager to see the beta.