Here is my big comparison which took me forever to get around to. Complete with commentary on each archaic. I haven't used every single one of these before (in particular necro/druid) so if you have any more insight on a particular archaic post it up.
Note: For the replacements I am assuming that the target has equal resists across the board. Obviously there are situations in which a particular spell would be better than the archaic due to lopsided resists.
Note2: I threw in the damage/mana for the nukes just as a comparison. Actual damage/mana differences will be more noticeable due to focus effect, cunning of the beast, damage increment, etc.
Cleric
Archaic: Avatar of Mercy
Long Duration Self Buff
Increase Healing by 15%
Increase Aggression Multiplier by 75%
Increase Maximum Mana by 150
Replaces: Vessel of Althuna
Level 65 cleric spell, going rate between 7-15k depending on supply/demand
Long Duration Self Buff
Increase Healing by 10%
Increase Aggression Multiplier by 80%
Difference: 5% more healing, 5% less aggro, and 150 mana.
Overall Usefulness: A nice little self buff. The fact that it eclipses an expensive level 65 spell is a little rough, but its not too big of a deal I guess, most clerics who get Archaic have already had Vessel for a long time and got good use out of it.
The -aggression is nice and all, but not really a big factor. Clerics basically never pull aggro, especially with foelock in. The 5% healing increase is definitely a nice little boost too. 200 more hp to my 5150pt heal. About 100 more hp 2650pt quick heal. Its good, its useful. Its not raidchanging. My day to day experience is pretty much the same. Having the Archaic doesn't make my healing reasonably easier, or even noticably different from a cleric without archaic. As explained above, the 200 extra hp is not worth too much in most raid situations. The 150 mana is probably worth more, I am guessing it gives me at least 1 more LoE.
Shaman
Archaic: Spirit Sleep
Single target magic based slow with a -100 adjust
Lasts 5.5 minutes and costs 265 mana
Cast Time: 1.0 second, no recast
Decrease Attack Speed by 55%
Increase Disease Counter by 20
Replaces: Turgur's Insects
Level 51 shaman spell, sold on a vendor
Single target magic based slow with no adjust
Lasts 5 minutes and costs 250 mana
Cast Time: 3 seconds, 6 second recast
Decrease Attack Speed by 50%
Increase Disease Counter by 16
Difference: Casting speed is decreased by 66%, uses 15 more mana, has a massive -100 resist adjust and goes from 6 second recast to no recast.
Overall Usefulness: Extremely useful. Makes botting a shaman about 5 times easier. Gone are the days of waiting for the shaman to malo, mage to malosini, enc to tash, and then wait a full minute while your shaman gets 4 resists and your tank gets destroyed. Basically removed everything that sucked from turgurs to create the uber slow. This is possibly the best archaic.
Druid
Archaic: Tarhyl's Raging Curse
Single target fire based debuff with a -100 adjust
Lasts 10 minutes and costs 275 mana
Cast Time: 3.5 seconds, no recast
Decrease Armor Class by 300
Decrease Fire Resist by 100
Decrease Attack by 100
Increases the damage of fire nukes by a decent percentage, we don't have one yet.
It also might affect all nukes? The only person with this is Caelin.
Replaces: Enclosure of Tarhyl
Level 62 druid spell, not sure on the going rate but probably in the 150-500p range
Single target fire based debuff with no adjust
Lasts 10 minutes and costs 325 mana
Cast Time: 4.5 seconds, 6 second recast
Decrease Armor Class by 107
Decrease Fire Resist by to 88
Decrease Attack by 107
Decrease Current Hitpoints by 200
Difference: 193 more AC debuff, 12 more FR debuff, 7 less attack debuff, 50 less mana, 1 second faster cast, no recast time. And the icing on the cake is the damage increment focus (probably only affecting fire nukes), which might be equivalent to about DI3 or DI4 from what I've heard. I'd love some confirmation from forsaken wizards/mages/druids.
Overall Usefulness: Well I can't comment too much on this yet. We definitely do get Enclosure of Tarhyl in on every raid encounter, and this is a decent buff to that. Plus it adds in that damage increment, which, even if it only affects fire, is awesome for mages and wizards on many encounters. Adds a really nice additional layer of desirability over Enclosure of Tarhyl.
Necromancer
Archaic: Claws of the Chill
Cold based single target dot with a -200 adjust
Lasts 4 ticks and costs 480 mana
Cast Time: 3.75 seconds, no recast
Decrease Attack by 50
Decrease Hitpoints by 645 per tick
4 Ticks X 645 = 2580 for 480 mana = 5.375 damage/mana , -200 cold adjust
Compared to:
Relic: Marlows Cremation: 6 Ticks X 420 = 2520 for 465 mana, 5.42 damage/mana, -125 fire adjust
Scitterpox: 9 Ticks X 290 = 2610 for 525 mana = 4.97 damage/mana, disease based, no adjust
Funeral Pyre of Malath (lvl 60 bought spell): 6 Ticks X 330 = 1980 for 450 mana = 4.4 damage/mana, -100 fire adjust
Caress of Sivyana (lvl 62 dropped spell): 215 + 4 Ticks X 498 = 2207 for 505 mana, 4.37 damage/mana, poison based, no adjust
Relic: Spirit of Kaezeul: 5 Ticks X 360 = 1800 for 520 mana, 3.46 damage/mana (lifetap), -200 magic adjust
Difference: Its a little strange that it looks like Marlow's Cremation is more damage/mana, but I guess you also have to consider that the Archaic has better resist adjusts, the attack debuff, and it is faster. Not to mention the Archaic would actually probably end up replacing Funeral Pyre or Caress of Sivyana in the spell lineup since necros stack up a ton of dots. You can see the differences between them above, not typing them all out.
Overall Usefulness: Replaces one of the crappier necro dots to increase their DPS. We don't have one yet so I can't comment on how massive it is for necros. The attack debuff coupled with a decent increase in DPS is pretty sweet and should cause a reasonable increase in the DPS of a necro who gets this. I know Paxx is dying to get his.
Enchanter
Archaic: Vapor Trance
Magic based single target mez with a -35 adjust
Lasts 3 minutes and costs 175 mana
Cast Time: 1.5 seconds, no recast
Mesmerize (Level 65 max)
Replaces: Complacency
Level 62 enchanter spell, going rate probably between 200-500p
Magic based single target mez with a -25 adjust
Lasts 2 minutes and costs 160 mana
Cast Time: 3 seconds, no recast
Mesmerize (Level 65 max)
Difference: Has 10 more resist adjust, uses 15 more mana, lasts 50% longer (another minute). Also cuts the cast time in half.
Overall Usefulness: This one is probably one of the weaker archaics. Its a decent improvement over the older mez, but it doesn't really stomp it. The casting speed is the best improvement by far and I am sure enchanters are very happy to get this. There are probably only a few select encounters where this spell is actually useful on a raid however. But I know its sweet for groups.
Wizard
Archaic: Moon Comet
Cold based single target nuke with a -20 adjust
Costs 525 mana
Cast Time: 5.75 seconds, no recast
Decrease Hitpoints by 2575, 4.90 damage/mana
Replaces: Relic: Tarhyl's Embrace
65 Wizard Relic Spell (clearly not really "replaced")
Fire based single target nuke with a -10 adjust
Costs 510 mana
Cast Time: 6 seconds, no recast
Decrease Hitpoints by 2250, 4.41 damage/mana
Difference: You can see the damage/mana is a notch up on the Archaic. Offers a high damage cold nuke instead of fire.
Overall Usefulness: The biggest, baddest nuke in the game. Gives wizards the ability to actually do DPS on things that are resistant to fire, and get even more massive crits on stuff that is equally resistant to fire and cold. Definitely a very desirable archaic that every wizard longs for.
Magician
Archaic: Sun Storm
Fire based single target nuke with a -20 adjust
Costs 375 mana
Cast Time: 5.75 seconds, 2.5 second recast
Decrease Hitpoints by 1745, 4.65 damage/mana
Replaces: Relic: Bladewind
65 Magician Relic Spell (once again not really replaced)
Magic based single target nuke with a -25 adjust
Costs 350 mana
Cast Time: 4.5 seconds, no recast
Decrease Hitpoints by 1520, 4.34 damage/mana
Difference: Has better damage/mana. Gives mages a nice big fire nuke over the Relic magic nuke.
Overall Usefulness: Another weak archaic, but this has been brought up a whole bunch. It is definitely more damage per mana, but due to the casting time Relic: Bladewind is actually more DPS. Its only desirable for encounters where the mob is MR but not FR. Needs some tweaking, but thats for another thread.
Comments:
So we have to look at how powerful an Archaic should be. I think it should be good enough that it distinguishes a character that doesn't have it from a character that does have it by a decent amount (ok that's really subjective, but these archaics are all totally different depending on class). Currently I think that at least the mage and cleric, and possibly enchanter Archaics do not really do this. (Well the enchanter on is up for debate, its not that useful for most raids, but very useful for grouping, that's another thread though). I definitely want a shaman, wizard, necro, and druid with their archaics over those without. But for clerics, my mana, FT, hp, and resists are overwhelmingly more important than my archaic.
There have been some decent suggestions in this thread. My number 1 limiting factor on raids is running out of mana from casting LoE. I know effects can be added that only effect specific spells. If not a blanket mana conservation effect, perhaps a 15% mana conservation for LoE? Or you could add other stuff such as FT that isn't affected by the cap, more hp, more mana, death safe chance, and/or resists. Finally, this one is a little out of the box but maybe a targetable death save replacement for divine benevolence with a 75% chance of working, that you can only cast every 15 minutes and has a 15 minute duration, or something along those lines.
EDIT: Also on comparing nukes versus heals, you have to keep in mind that a nuke that does 10% more damage/mana is worth SO much more than a heal that does 10% more healing. The goal of heals is to prevent the tank from dying, and with multiple healers all healing at the same time, the 10% more healing doesn't have a very big impact. While doing 10% more DPS has a much larger impact. This is why something like a mana conservation so I can heal for longer or high chance death save for when something goes wrong would be a lot more valuable.