In the early stages of the game, Priest PvP healing wasn't as much of a specialized spec as it is today. With a game and itemization centered around PvE (pre TBC), Holy was a natural go-to as raiders would just bring their raiding specs to the Battleground and healbot their Dreadnaught clad Arms Warrior buddies after a night of beating back monstrous dragons and their progeny.
However, as time passed, through tweaking by Blizzard, the Discipline tree has been gaining popularity as the default PvP tree for utility and flexibility. Early Arena seasons saw hybrid Disc/Holy specs such as the old standby 28/33 for Blessed Resilience. As gear started to match damage in later seasons, 41/20 Pain Suppression builds started showing up en masse with Priests electing to give up some personal survivability for added situational answers.
To try to predict where all of this will go in the months and Arena seasons to come, it becomes key to understand the natures of the two healing trees, where they excel and where they fall flat. Keep in mind that the Priest is the only class that has two healing trees, so it's pretty unique that there are viable options for a class to spec when dealing with a variety of situations.
There's a couple of overall differences between the functionality of the Discipline and Holy trees in PvP. When comparing the two functions, I find that the best term to describe differences is "Bandwidth vs Throughput". The analogy between the characteristics of these Priest talent trees and the traditional usage of these two terms in the data transmission can be confusing at best, so let's break it down for non-technical people out there.
First, to establish the boundaries of the explanation, there is usually a limiting factor. In data transmission, the limiting factor is usually cost, and to a lesser extent, amount of data to be transferred. Simply, if you're willing to pay for wires that can handle more data, you'll get better performance. To dumb it down even further, we can think of wires as plumbing pipes for data and make the observation that narrower pipes typically cost less than big honking pipes (for proof, go visit Home Depot). So basically, if you have to move .. say .. a large quantity of .. excrement (or Porn), but you're limited in the amount you can spend on the pipes, your options are either one big pipe or several small pipes. The same is with data and (hopefully you're beginning to see the parallels) with healing. (And by using the word 'healing' we're essentially referring to death prevention)
The point, without getting mired in .. crap .., is that these two characteristics are generally proportional. Within the boundaries of cost and amount of data (mana and time alloted to 'save' someone) you can either cast a big heal or several small ones. It stands to reason that the Holy tree is the spec for throughput in that it puts out some really really big casted heals, typically with a higher crit rate, and possibly boosted by procs of additional big heals or even BIGGER heals. By contrast, the Discipline tree represents the bandwidth aspect of death prevention through its effective shields, cheaper dispels, extra haste, shield proc effects and several utility cooldowns.
Interestingly, in PvE, a typical raiding Holy spec can represent both 'bandwidth' and 'throughput' with just a shift of talents, however, this is not really the point of the discussion so we'll leave it for later (if people are even interested in this spec's discussion).
Ok, so where is this all going?
Well, predictions, of course.
Well, as with Arena season 1, the initial set of gear for many fresh level 80's will likely be pretty poor. A mix of PvP and PvE gear will be pretty prevalent and the PvP gear that is present will likely be fairly weak and unable to hit the resilience cap. We will likely see many double DPS in 2's and triple DPS teams in 3's as healing (due to gear) is very unlikely to match up with the potential damage output. In addition, due to poor weapons and low stats, melee is unlikely to perform as well as casters, making caster teams very viable.
As such, with knowledge of past Arena seasons, a Holy spec is likely to be the dominant spec for the variety of 2's and 3's comps where the Priest is the solo healer. I'm not saying that it will be the only spec for solo healing Priests, but it is unlikely that Discipline will have the throughput and personal survivability as compared to Holy to deal with the vast amounts of pure DPS teams out there. Once the healing starts catching up with the burst DPS and more Healer/DPS, Healer/2xDPS and 2xHealer/DPS teams become viable due to better gear becoming available, it's logical that Discipline variants will become more and more popular. This is primarly due to the better efficiency and 'tricks' that are needed to set up a kill on teams that are designed to survive for more than a few minutes
Of course, comps such as Rogue/Mage/Priest or other similar incredibly offensive burst comps would probably benefit from a Discipline Priest over a Holy Priest due to the Haste effects, cheaper Dispels and 0.5 second Mass Dispells for all of those incredibly annoying Divine Shields and Ice Blocks out there. We'll be sure to do a write up on their viabilities once we get to 80 and nab us some gear.
As for right now, we still have 30k honor to grind.