Priest specs for arena - Holy/Discipline

The usual spec for a healing priest is 28 discipline / 33 holy because it can be used for all brackets and make it hard to kill the priest. It can be used for PvE without losing much healing and efficiency. Deep discipline specs are also viable to heal but they're better for 2v2 than other brackets. It's not very good for PvE and requires +damage gear to get the most of it. I'll post about other specs later.

Starting from discipline.

• Unbreakable Will - Get it, no need to explain.
• Silent Resolve - This may prevent other priests and shamans to dispel some of your buffs. The more time and mana they waste dispelling the better. It's also helps for PvE.
• Improved PW:F and Improved PW:S - Some people get this, I don't think it's good enough. You can put some points here and remove points from Silent Resolve.
• Martyrdom - Considering they'll focus fire on you half of the games and mostly by warriors and rogues, this is a good talent.
• Absolution - There're 2 things a holy priest can do in arena other than healing, dispel and mana burn (well.. we can try to run too..). And you want to dispel HoTs, shields, Blessing of Freedom and all their buffs. Mass dispel must be dispelled fast if you go against a paladin. So.. get this.
• Inner Focus - You get OOM, this helps. It's good for a one point talent.
• Meditation - No good for arena because we don't have enough spirit with arena gear to make it worth. BUT if you play some PvE you'll want it. I get it so I can raid without problems.
• Improved Inner Fire - Increases armor against melee and ranged damage. It's 474 more armor (I think it's ~4% less damage). It's good for PvP, useless for PvE most of time. If you only play arena, get this.
• Mental Agility - Must have, most of your spells are instants, including: Renew, PW:S, PoM, Dispel, Shadowfiend, SW:D and SW:P.
• Improved Mana Burn - Another must have. In long games mana is what makes the difference. Drain their healer's mana and your team wins.
• Mental Strength - Not good enough. With 8.5k mana this gives 850 mana and it's a 5 points talent. Mental Agility is better.
• Divine Spirit - Another buff for the other team to dispel and will help you regen a bit of mana. If you already got Meditation, get this.
• Improved Divine Spirit - Only useful for raids (even for raids I wouldn't get it), completely useless for arenas where not even a priest has enough spirit.
• Focused Power - You need this to dispel paladin's bubble and mage's ice block. Considering the amount of paladins on all brackets...

Holy tree

• Healing Focus and Improved Renew - Get them.
• Holy Specialization - I get 5 points here, but you can't rely on crits, most people just get enough here to get talents lower in the tree.
• Spell Warding - Good talent, not a must have but it's nice to have.
• Divine Fury - Must have for PvP and PvE. Some says you never use Greater Heal on arenas, but this isn't true. I use it a lot because of it's mana efficiency.
• Holy Nova - I just get this to kill snakes from hunter's trap. You can also spam rank1 of this against stealthed opponents.
• Blessed Recovery and Inspiration - As holy specialization, you get this to get other talents. Some people get one point on each just for the proc, another buff for dispel.
• Improved Healing - Same as Divine Fury.
• Healing Prayers - You'll use PoM a lot, so this one is good but I don't see as a must have.
  
• Spirit of Redemption - Ah.. improved death. This one is really good for PvP. Free heals after you die can win games especially against DPS only teams on 2v2 and 3v3.
• Spiritual Guidance - Don't get this for PvP. Even for PvE you need a LOT of spirit to make this worth.
• Spiritual Healing - The best talent to get so you can reach Blessed Resilience and 10% healing is really nice to have.
• Surge of Light - Waste of talent to get this. You don't have enough crit to get this to work and discipline talents are better.
• Lightwell - Lolwell... completely useless talent. IF they eventually change this so it doesn't cancel the effect when you get hit, this will be useful with improved death (I never got this, so I'm not sure you can cast this with improved death).
• Blessed Resilience - The reason holy tree is better than discipline tree even for PvP. If you do arena, get this one.
• Holy Concentration - You don't have points to spend here... It's a good talent though.
• Empowered Healing - Gives +200 healing to your Greater Heal and +100 healing to your Flash/Binding Heal if you have +1k heal. To get it you need 3 more points in holy. This talent is not worth for PvP, it scales with +heal and you don't have enough when you use your sockets for +stamina and +resilience gems.
  
• Circle of Healing - Some people get Circle of Healing for PvP. I tried it when I was lvl60 and it's good. CoH is a nice spamable spell that is probably better than PoM (with +1k heal it should heal for good 900 and cost 450mana). But it has 2 problems, 15 feet area is too small and 0.5sec Mass Dispel is better than CoH and Empowered Healing. With PoM I don't think you really need it.

How to play with this spec depends on your team and bracket. For 2v2 and 3v3 and you're the only healer, you'll probably heal more than anything else, but this also depends on your team style and the opposing team matrix. Defensive games needs more heals and defensive dispels. You should also stay back and keep yourself out of their line of sight if possible (so.. less fear). Offensive games requires offensive dispels and mana burn, you also need to fear more. Pillars are your friends, use them. Be aware that, if your team's target is targetting you, you kite him, you also kite your team. Mana burn should be used against healers and shadow priests, but mages and hunters are good too. Drink when possible and have your team pets on healers so they can't drink. Shadowfiend should be used when you get to ~50% mana, so it also helps with DPS.

It's usually a healer task to kill totems when you go against shamans (unless you're their primary target and have to kite them). Use melee weapon to kill totems (usually grounding, tremor first then windfury), it's faster and don't get global cooldown (use wand for earthbind or far away totems). Also try to avoid hunters LOS ALWAYS. Viper sting makes you useless. Try to pay attention to Blessing of Freedom and bubble if they have a paladin. Blessing of Freedom is one of the reason you should keep their warrior without buffs.

For 5v5, unless you're the main healer, keep dispelling and mana burning. ALWAYS keep a PoM up. Kiting is a lot harder here if not impossible for some games. Don't get on their warrior LOS. Without intercept or rogue it's usually hard to get the priest. Defensive dispels are usually for CC, dispelling DOTs only after dispelling the other team. A mage can use Spell Steal if they know what they'll steal. This is why my dispels are slower to remove sheep when I'm on a 5v5, usually I'm paying attention to opponents bars/movements, differently from 2v2 and 3v3 where I try to pay attention to my team bars/movements.