1. Holy is somewhat problematic to judge. On the one hand, this is finally the expansion where they haven't been outshined by Disc, especially in final tier. They bring respectable hps, that's useful in pretty much any encounter and ... well, that's it. It feels a little too basic, you charge your Holy Words with filler spells, use them, go back to spamming filler. It's also the only spec with non-smart AOE heal and the only stationary channeled raid cooldown in the game. Also, our Artifact ability is... well, it exists. I think. It's hard to tell at times, since it's too random and doesn't really do much.
The spec is largely the same as it always was - no utility, just hps. Symbol of Hope was supposed to be that, but its' horrible placement in talent tree makes it useless. About the only "utility" we get is the Legendary Cloak, as it allows for some weird "soaking" or bypassing of certain mechanics. I'm going to miss that thing in the future.
Still, it's the big winner here. You could reasonably choose between two healing specs and not feel like you're holding your raid back - outside of some niche bosses that "need" Barrier, like Il'gynoth or Immonar.
2. I have mixed feelings about Disc. They finally dealt with absorbs being a "must have" and "too unique for their own good", by pretty much limiting it to PW:S on cooldown. On the other hand, the spec is frustrating for new players or without knowing the bosses. It relies on huge bursts of healing through massive cooldowns - which are bigger than any other spec has - followed by tediously reapplying your buffs one by one. Also, it took them quite a while to fix Disc reliance on Innervate.
3. Shadow... I find it the worst it has ever been outside of Vanilla. The numbers weren't the problem - it was absurdly overpowered early on and still does pretty well nowadays. But the design is horrendous. It's the slowest and most punishing ramp up in the game. Zero cooldowns outside of StM, which is too situational to count as such. One of the worst AOEs in game - and that's despite it being buffed by a huge amount, several times. Questing is a nuisance, as Voidform drops far too quickly to be useful in outside content - except for Argus elites. The fact that it's more efficient to respec and spam Holy Nova to complete some quests is both hilarious and sad at the same time.
Honestly, Shadow annoys me so much, because it's the first expansion where all three specs were open at the same time, so I thought I could *finally* have a decent questing/spare time raiding option. Instead, there's abomination like Mass Hysteria that cripples the spec outside of high end raiding and makes it a chore to play.