Priest would be the most powerful class in the game if you could create a deck specifically for the opponent you are playing. However, Hearthstone is a game that randomly pairs you with one of nine classes and demands flexibility. The current reactive play style and deep roster of situational spells makes Priest play highly susceptible to RNG. Most players claiming to have a high ranking priest deck have loaded up their decks with neutral minions and few priest cards.
Powerful, One Dimensional Spells
New players who come to complain about priests are often the victim of a one of the Priests powerful but situational or one dimensional cards. They remember the times that they finally played their "OMG Minion of Doom" that was going to turn the tide of the game in their favor just to have it mind controlled or Shadow Word Deathed. Cards like Mind Control, Shadow Word Death, Shadow Word Pain, Silence, and to a lesser extent Cobal Shadow Priest, Inner Fire, Divine Spirit, Holy Nova and Mass Dispel can turn the moment of a game around when used at the right time but can also have the potential to just sit in your hand doing nothing sometimes for games on end. Some of these cards do not even have trade down potential.
The Most Reactive Class
Should be no surprise to people that the class themed around Healing and Defense is the most reactive class in the game. Allowing the opponent to set the tempo of the game is very dangerous in this game. Given the Reactive nature of most of the priest cards you have to hope to have the right cards at the right time in an almost endless pool of different scenarios. Decks loaded up with priest cards/combos are highly dependent on card draw and specifically Northshire Cleric.
Priests, especially when it comes to dealing with minions with 4 attack, are forced to use multiple cards or more expensive cards to eliminate the threat. A common scenario is Inner Fire + Shadow Word: Pain/death, Minion + Smite, or 2 minions. Trading down a Six Mana Holy Fire for a 4/5 Mana minion that has been on the board for 1-3 turns. A Ten mana Mind Control for 4-6 Mana minion. With few exceptions most of our cards are traded in a best case scenario as 1:1. Cards like Mind Blast and silence are not even worth a full card of value outside of a few scenarios.
Forum Observations: Neutral Minion Spam
How are people getting around these issues now? Players who claim to have broken into the top ten rankings typically run with Nuetral minion decks with few (often less than ten) priest cards. These decks trade cards like Holy Nova for Abomination, Inner fire/Divine Shield for Dark Iron dwarf or Defender of Argus. High defense mobs for mid-ranged mobs. Silence for Owl or Spell Breaker. Basically they are using a cookie cutter deck approach and priest just for the Class Power and not specifically a Priest deck.
Priest does not need a buff or a nerf at this point. Cards need to be retooled to be more flexible. Priest itself is a very inconsistent and frustrating class to play without subbing out all your priest cards for more flexible neutral cards.