There are a few reasons why positioning is important for the priest, more than other classes. For one, the priest is a highly vulnerable class. It does not have many defensive cooldowns like other classes (if a hunter is in danger, he can use Deterrence, or Disengage, or Feign Death; A rogue an use Vanish, Evasion, Sprint, or Cloak of Shadows. A warrior can Charge, Intervene, Shield Block, Spell Reflect. . . . . etc.) A priest does not have many abilities like this to gain mobility or remove incoming damage. A priest has only Dispersion, and it is a 1.25min cooldown. So, we cannot correct errors as easily as other classes, and our positioning on the battlefield has more consequences than other classes.
Secondly, the priest has a responsibility to Dispel Magic in the arena match. And he cannot Dispel Magic if he is CC'd. So the priest must position himself so that he avoids damage, and avoids CC spells, in order to make full use of his spellbook (enemies will want to CC you because they know if they CC your teammate, you can dispel it, but not vice versa). Your rog does not care if he gets CC'd because you can dispel it. He can run around. But if you get CC'd, your team suffers.
2. HOW YOU SHOULD POSITION
The basic position is to stand beside an object. The object allows the priest to hide from enemies if he needs to (see image below), removing line of sight. A priest can hide from hunter so that he cannot be DPS, he can hide from a mage so he cannot be polymorph.
There are a few different scenarios, and in the future I will update this guide further to reflect the scenarios. But for now I will just start the post and leave the photo below.
3. MAP EXAMPLES
CLIP: https://www.twitch.tv/connoram/clip/...LEJZohnnTGKx9o
In the above clip, you can see that I hug the object so that mage cannot cast on me. I am casting support spells on my DK, and offensive spells on paladin. When mage cast Blink to get LoS on me, I know CC is coming so I back up and run around corner to cancel his polymorph cast. When paladin comes to me, the first spell I use is Shadow Word: Death. I only stand still to cast spells when I am not at risk of being polymorphed or counterspelled.
Sooo, in last screen hunter can attack rogue, pal can attack rogue, but you can't cast anything on anyone, unless you will expose yourself to the hunter. So the question is: what's next move?
In these situations it becomes cat and mouse. You hold an angle till hunter commits deep to shut it down. Then you move to other side of pillar and you have recreated the same as photo #1. If hunter stands directly between you and rogue he is a kill target, and you have benefitted the game as much as you can from your positioning. You can see how mage becomes the target when he positions like that in this clip: https://www.twitch.tv/connoram/clip/...LEJZohnnTGKx9o And the more they pressure you by having LoS on you, the more you need to only use instant cast spells and peek like you see me do in this clip.
Also remember: its not like this is a freebie for the hunter. The whole point of priest positioning is to make it as expensive as possible for the enemy to get to you. Hunter has to keep playing the arena while chasing you around and it reduces his damage unless he's one of the best hunters. He also is forced to go beside your rogue where he can get cc'd or attacked. You create opportunities by this position. As an aside, LoS naturally pulls people towards you, and you get opportunities to use AoE fear better.