The traditional role of Holy Priests (from the start of WotLK to the end of MoP) used to be their ability to go all out, reaching much higher max Hps than any other Healer, with CoH/PoH spam, while also emptying their entire manabar over 30sec, while being a slightly weaker Resto Druid when they were not going all out.
However with everybody just spamming Renew the entire time in WoD, Holy Priests seems to fill exactly the same role as Resto Druids, with a few differences to their kit, CoH/WG, DH/Tranq.
Oh, and Prayer of Mending.