Glad to see another player return.
PvS is a truly great set, and as far as I can tell from my games with it, impeccably balanced. The Paladin and Siren represent probably the most unique mages to date, and the kind of games they encourage are very memorable.
That said, while I feel that the Siren is definitely fleshed out to good degree and a great addition to the mage roster, I think fans of the Holy school in particular gain an enormous amount from PvS. If I had to nominate one card from PvS as the biggest game-changer, it would be Healing Madrigal; expect to see that card in many army/swarm builds to come. Especially with the Siren, it's a total workhorse and in general keeps armies marching long past their typical expiration date.
Temple of Meraveran is a close second, as it's a literal dice machine when armies clash. The Holy school didn't really support a swarm strategy offensively before the ToM, but it now does a great job of it with that temple, the holy terrain, and the Dawnbreaker Initiates.
Pillar of Righteous Flame is a great anti-swarm epic card that every mage has access to and allows mages to fight more effectively against Necro armies.
Holy mages also get a slew of Holy-mage only cards like Radiant Breastplate, Cassiel, Martyr's Restoration, etc, that give a ton of character and powerful options to the Holy school. The whole idea of a temple build is also viable now that there's a lot of temples, Vigilant creatures, and Metatron.
I thought the Holy school lacked some depth before PvS, but I can safely say that after its release, that's definitely no longer the case. PvS brings so much to the table for Holy and Water, and even introduces Elephant grass to aid Beastmaster builds that aren't bird-based. 10/10 expansion, they really did make those 2 years count.
