Mind controlling other players was, since vanilla, perhaps the single most truly unique signature capability of a priest. And now, apparently, it is gone -- not an option to talent into part-time but gone from ever experiencing in any form of PvP.
I know such is intended to permit this build's PvE buff, but they could have had the new baseline PvE-only ability version while making the talent choice instead read "Dominate Mind affects player targets. On non-player targets, you may control your own character while Dominate Mind is active." That would allow the PvE buff while keeping the PvP situation the same as now.
Spectral Guise was the most outright fun addition to the priest class in several expansions but now also getting removed (and that isn't to prevent stealthed fear bombs, as could have been done simply by making Psychic Scream not usable for several seconds after exiting stealth).
(Mana burn once ranked up there in uniqueness too, albeit not as special in fun, but it is long gone).
Those added interest and flavor in a way a bunch of effectively +X% to HPS or DPS talents do not as much, with the latter barely visible outside of number changes. Maybe some players get excited if they see +20% to the healing of something, but, from a perspective of less naivety, HPS is nearly a constant anyway after balancing to make all healing classes equivalent in HPS terms. For example, +10% from one talent really just means -10% in base numbers or otherwise.
Especially since all healing classes must be about equal HPS in the end, where the most interesting differences between classes can exist is rather in utility (and arguably fun) ... and priests just lost a huge chunk of theirs in PvP, especially world PvP. There are some unique fun capabilities or special cool aspects outside of simple healing/damage left (like I'd consider Leap of Faith, Angelic Feather, and holy-spec Spirit of Redemption so), but a large chunk is already lost as of Legions beta.
Other than healing/damage itself, Legions sets priest utility / special fun and CC at the low end of the healer spectrum, closer to healers in the average less-popular-than-WoW MMORPG, much below the high end (WoW druids with stealth and a multitude of CC). The low end is not necessarily the model to aspire to. There is a reason I don't play a paladin or shaman healer; I always found them relatively less fun.
Blizzard doesn't market its expansion packs by labeling them "deletion packs" or "mixed expansion-deletion packs." There was actually almost no deletion of anything from the game in its first half-decade, until post-WOTLK, and it survived rather well. WoW only went up in subscriber count in that era, before the more bittersweet post-WOTLK era of one having to wonder as much what will be lost as gained each expansion. (I have a suspicion that my fun from engineering tinker enchantments in battlegrounds and world PvP is going to get removed too, in the name of a balance goal that could have been sufficiently managed just by near-equalizing item level instead).