They're everywhere. Why?
Let's start with paladins:
SA: Take damage (get healed), gain mana. It's a weird variant of the rage generation from taking damage.
JotW: Deal damage (judge wisdom), gain mana
BoS: Block (dodge, parry), gain mana
Focused attacks: crit, gain energy.
Combat potency: 20% chance for off-hand attacks to generate 15 energy.
Setup: take damage (dodge or resist), gain combo points. This one, as best as I can remember, has been around since pre-BC, at least a year before then. So I suppose it breaks my attempt to create a pattern of new rage-like mechanics. Let's just sweep this under the rug... *sweeps*
Dead damage (use runes), gain runic power.
Anit-magic shell: Take damage (mitigate 75%), gain runic power
Invigoration: gain mana when pet crits
Go for the throat: pet gains focus when you crit
Aspect of the Viper: attacks generate mana
Incanter's Absorbtion: increased spell damage from taking (absorbing) damage
Magic Absorbtion: Mana gain from taking (resisting) damage
Frost Warding: The description confuses me, but somehow you gain mana from people trying to kill you.
Shamanistic Rage: Deal damage, gain mana based on AP
I might have missed some. I definitely stretched some of these a bit. But the trend is there: dealing or taking damage is becoming a normal source of regen. Previously there were three types of regen which operated much differently.
Energy was a constant regen, increased damage came from only increased damage per point spent.
Mana had scaling regen and increased damage could come both from greater damage/mana and from just plain having more mana.
Rage was all over the place, directly scaling with damage stats and also increasing damage/rage with greater stats. It was unique in also scaling with damage taken, due to its dual purpose as a resource for both DPS and tanking.
Why the spread of rage-like mechanics? Well first off, they offer a distinct advantage over pure mana: they last longer, potentially forever. Oh sure if you stand around a while you'll regen mana, but except for strange times like now with loads of regen, mana bars have generally been things which move from full to empty during combat, not the other way.
Isn't this an intended weakness of mana? Well sure, but that doesn't work so well when you try to put it in other roles. SA was critical to paladins having any chance of tanking. They somehow needed a lot of regen (tanking is expensive and the armor has poor mana, even when we had int on tier sets) which wouldn't be too powerful for the healing and DPS roles. SA did that. BoS was added because SA was scaling negatively with gear and while there is a balance to be struck between aggro and mitigation, they were in direct conflict.
JotW was a failure in a way. It was a failure to work within existing mechanics (switching between SoW and a damage seal) to create any type of burn-regen cycle, instead just sticking mana into the normal 'rotation' and declaring it solved.
Rogues? No clue why they got the mechanics. Maybe Combat Potency was to make hit and haste more popular. Maybe it was just to catch the attention of the fabled Rogue with One Dagger. Focused Attacks: it's a crit tree and eventually you can't add any more crit increases or crit damage increases and they already had combo points, so they added energy. Any similarity to rage seems unintended (not to say, "not working as intended", just that they weren't trying to give rogues rage) and ultimately these scaling regen sources still end up being less than the static regen, at least at my poor lowbie rogue's gear level.
Death Knights: I realized that DKs don't really use a rage-like system, it's really more like energy and combo points. Runes are the energy: static regen. Runic power is the combo point: a resource generated by spending energy for some other purpose, potentially scaling with talents or gear but ultimately still restrained by the static part.
Hunters: See ret paladin (except done correctly). Also see rogue (the part about running out of mechanics to add).
Magi, well, they're known for their talents which sound cool but frequently have no practical use. Woo, armor from intellect, they are going to be untouchable to melee! Oh. Well at least the absorb -> SP worked well before they fixed the exploits *cough cough* soloing Military Quarter.
Shamans: See ret, but done better.
I am left wondering something: why do shamans and hunters have scaling regen? This makes balancing much more difficult. Now you can't just lay out a rotation and see what damage it does. Now you have to consider whether they are able to sustain that rotation (scaling goes both ways) or if they're actually expanding that rotation to something more expensive which they can still sustain. It's rage-like and as a result that much harder to model.
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