Since writing this post and the release of 3.1 Exorcism has been hotfixed to no longer be useable on PvP targets. Read about it here.
You'd probably have to have been living under a bush to not hear about the proposed 3.1 changes. One, the nerf to Divine Plea, has already reached a live patch in the form of 3.0.9. However the most controversial Paladin buff is certainly the lifting of restrictions on Exorcism. This post was going to discuss how non-Paladins were stating that it will be overpowered without taking other anticipated 3.1 changes into account, but instead I'll mention some of the likely changes for 3.1 to this spell.
Of course, the primary reason why Exorcism is being brought into the fold for DPS is the need to sustain Ret DPS moving from an Undead raid. Exorcism and Holy Wrath combined contribute to around 7% of Retribution DPS, and including the Sense Undead Glyph the simple migration to Ulduar would reduce Ret DPS by 8%. This is pretty untenable in any sort of min-maxing environment without unique buffs, and the change to Exorcism will ameliorate this. As a fun side effect it becomes useful for Holy (which has woeful DPS) and allows designers greater breadth in Talent design. Unfortunately, this change may come at a price.
Exorcism is extremely mana efficient for this type of spell.
Exorcism is most similar to Shaman Shock spells, the odd Mage spell or Shadowburn. Its target restriction feature was used as rationale for increasing the damage per mana of the spell some time pre-2.0 IIRC, and in early Beta the mana cost was again reduced. As of patch 3.0.8 it costs 8% of base mana (7.2% with Benediction) which is 10% less than equivalent spells. As instant cast damage spells go only Ice Lance is cheaper at 7%.
However a different comparison may be made. Hunter Shots are similar to instant cast spells excepting their minimum range and reagent requirements (reagents not required in in 3.1). Their mana cost is in single figures: 6% for Arcane Shot, 7% for Explosive Shot, 9% for Serpent Sting (if we expand the definition somewhat).
I don't think that there is anything inherently wrong with an 8% base mana cost for Exorcism, it's just that Blizzard may have to tie themselves into knots to justify it. Depending on the approach Blizzard take Exorcism's base mana cost may well be increased, possibly to around 15-18% in line with most other instant nukes. If this occurs I would expect a talent similar to Shamanistic Focus in the Shaman Enhancement tree (which reduces Shock mana cost by 45%) to appear somewhere low in Retribution or Protection to reduce the mana cost down to a more reasonable value (i.e. back to 8%). Anything higher than 10% probably isn't sustainable in Ret or Prot's rotation.
The Sanctified Seals talent, which previously made Seals undispellable, is a prime candidate for this.
High Base Damage
Exorcism hits hard. At an average damage per cast of almost 1.1k it hits considerably harder than most other instant cast nukes, though it should be noted that most equivalents have secondary effects at baseline or scale better. Flame Shock deals similar damage, but splits the damage between an instant component and a DoT. This has significant implications for PvP, much less so for PvE. In order to retain the DPS level seen in Naxx 3.1 Exorcism needs to inflict damage equivalent to 3.0.8 Naxx, but this need not be in one large lump.
A 1.1k nuke, coupled with Ret's already decent burst damage, may well be more than Blizzard are willing to implement in PvP. It may also be just too powerful, though this depends greatly on the PvP burst 'fixes' we'll be seeing in (or before) 3.1. If there is a change it should be similar to Flame Shock rather than a straight damage nerf, and hopefully the damage it deals won't be too closely tied to any talent tree.
It shouldn't replace Consecration in the rotation, as some are debating. A totally separate alternative to Cons needs to be created in order to preserve Naxx levels of DPS whilst replacing its' AoE component.
The 30yrd Range.
Okay, this is probably one feature of the baseline spell which may be a little over the top if it is brought through to 3.1. Shock spells are typically around 20yards in range so we may well see Exorcism brought down to this sort of range.
That Crazy Glyph
Adding a two second interrupt to a spell is probably one of the most powerful glyphs in the game right now if applied to all targets. That's not to say that a ranged interrupt on a low cooldown is overpowered - Earth Shock, Counterspell and talented Deadly Throw are all low-cooldown interrupts with the same or longer school lock-outs than Glyphed Exorcism would provide. It won't make a huge difference in PvE, mana conservation and +damage glyphs are much more important for Ret, and the 17% Spell Hit cap makes spell interrupt's a little too unreliable in PvE for non-DPS casters. There are certain fights where an additional interrupt for any spec would be extremely useful (e.g. Kel Thuzad apparently) but these encounters are probably too rare to be of significant concern in PvE. Indeed, a glyphed interrupt may well be useful balance-wise in 25-man and 10-man encounters from a design perspective.
In PvP however it is almost impossible to state whether or not any single spec would become too powerful with a 15sec cooldown interrupt+nuke at the cost of a Glyph Slot. The changes that have been announced simply create too much of a void of knowledge from which to make an assumption. There are a few things we can surmise will occur in 3.1:
Lower burst damge. Historically Retribution has lived and died on it's burst damage potential, which is why it tended to work well in Cleave teams. Lower burst is a direct nerf to Ret's viability and the spec has little to make up for that. Longer matches. Perhaps this is an indirect consequence of lower burst but the longer the match tends to be the worse Paladins perform. We saw this in Season 2, when Paladin representation nose-dived and Resto Druid ruled the roost. In general other utility such as Purge and CC become much more important. Since then Holy's mobility has improved because of Sacred Shield and Holy Shock but it is still lower than instant cast HoT classes which will get ever stronger as the ratio of burst damage to health pools decreases. Higher Cooldown Hammer of Justice. Sorry to say it guys but 30sec cooldown HoJ will probably be a thing of the past for Holy and Ret builds, if for no other reason than the likely changes for Prot announced. Longer cooldown HoJ means less control, which again tends to have a disproportionate impact on the battlefield.
All the above said, even if the Glyph of Exorcism reduces damage of Exorcism by 100% it will still be a massive improvement to all Paladin specs in PvP. Even if they increased the cooldown and decreased the range of Exorcism the glyph as it stands would be damned useful. Even if they did all of these things and increased the base mana cost of the spell it gives Ret and Holy much needed tools for PvP going into 3.1.
The point is that Exorcism and the Glyph of Exorcism may well change considerably with 3.1, but whilst its' mana cost isn't increased significantly and DPS is not reduced it will be fine for PvE (we still have plenty of spare GCD's for a 15sec cooldown spell). As long as the Glyph retains an interrupt component it will be very useful for PvP. We shouldn't expect the spell to remain the same but we can expect the spell to be damn handy if we take any sort of wider view.