It's been a pretty wild ride for Smite priests since the first look at the Cataclysm talent trees. The smite sub-spec has gone from a nice little bonus, to extremely overpowered, to probably better than using Heal, to totally unplayable and now I don't really know what to make of it.
The recently Archangel from returning 3% mana per stack of Evangelism to only 1% mana per stack of Evangelism. Since we are probably going to have a little over 100k mana, that a difference between 15k mana returned every 30 seconds and 5k mana returned every 30 seconds. Of course the difference is even more dramatic than that in a sense. Smite costs over 3.5k mana and a full Evangelism stack reduces that by 30%, of just over 1k mana. If you need to rebuild your stack then you need to cast one smite and full cost, one 94% cost, one at 88% cost, and so on until you get back to 70% cost. That means you are paying the cost of 30% + 24% + 18% + 12% + 6% of an extra smite, or 90% of a smite. That's around 3.2k mana. So in reality, using Archangel only nets you around 1.8k mana. It also costs you about half a smite worth of damage and healing. It also gives you a 15% healing bonus for 18 seconds, but then again it also costs you a GCD. On the balance it is a net increase in healing, but I'm not sure it's worth taking.
This, apparently, made the Smite spec unusable. The solution they came up with is as follows:
Smite's scaling coefficient has been increased. The damage of Smite is now very similar to Heal in value. This change has been made to ensure the Archangel Discipline sub-specialization remains viable.
There is a really glaring problem with this. Smite already had a far higher coefficient than Heal. If you make Smite hit for about as much as Heal in tier 1 raid gear, then by tier 4 heroic raid gear it is going to hit for quite a lot more than Heal. At that gear level, Smite has throughput much closer to Flash Heal than it does to Heal, and it has better mana efficiency than Heal. This is a very problematic place for it to be.
With the current numbers, tier 1 Smite priests are going to be a little sub-par and probably will have too many mana concerns, while for end-of-expansion content they will be very noticeably overpowered - providing around the same healing as another healer would by spamming their fast high-powered heal and doing about half the damage of a real dps character.
If mana was the problem for discipline priests, perhaps they should have addressed the problem by lowering the cost of Smite, rather than by increasing it's effect. The disparate coefficients of Smite and Heal were already heading for trouble. Pushing them further apart is going to be a bit of a catastrophe.