Priests are an excellent demonstration of what they are talking about.
I have been playing a priest since the game was released back in '04.
-- My priest was a Night Elf on the PVP server Laughing Skull. It was my main and only character for almost all of Vanilla. --
Priests were the undisputed champions of healing. Tank heals, raid heals, party healing were all easy and accessible to a priest with moderate skill and half-decent gear. Druids were great for topping off tanks and keeping HoTs stacked, but mostly they were brought along for Battle Res and Innervating the priests. Paladins were healing monsters for the few that knew how to play it, but they were best confined to tanks and offtanks.
-- I rerolled my priest to Human just over halfway through BC and re-levelled it up to take advantage of the 15% Spirit boost from the human racial. Fear Ward was globally available by now. --
The introduction of shamans and buffing of healing paladins to obscene levels stole the crown from priests as the definitive healing class. Druids had always been decent enough in their own right for basic purposes, but Burning Crusade gave them Lifebloom, and that is all they needed to stack with Rejuvenation to heal pretty much anyone. To balance this growing disparity of priests being relegated to raid-healing (meaning non-tanks) only, they were given Circle of Healing. Circle of Healing turned out to be a phenomenal spell. By stacking tons of spellpower and Spirit, a priest could effectively spam CoH across a raid and dominate the meters and keep everyone alive. Tank healing was left to Druids and Pallies. Shamans did both tank and raid healing pretty darn well. By the advent of widescale Tier-5 and Tier-6, priests were second-rate healers. We were brought along for CoH spam, and skill was no longer required. A good priest still used the arsenal of heals available to them, but let's be honest here... You really could just smash CoH to get through SSC and TK if you wanted to. Zul'Aman was probably the only raid instance that still required priests to do more than spam
Wrath Of The Lich King:
-- My priest was levelled to 80, did a few raids, then I took a few months away from the game. I came back to it after 3 months or so and still play it. --
Wrath introduced not only the scourge of the entire game, Death Knights, it also gave all of the priest toys away to other classes while giving none of theirs back to us. Druids got a Circle of Healing-type spell that expanded on their already ridiculous HoTs. Pallies were rocking the healing with Beacon of Light and glyphs that provide a basic group heal component to Holy Light. Shamans have had the resto tree buffed pretty much non-stop since Wrath release. Priests got... well... nothing. Guardian Spirit is really neat, but a 51 point talent that has no direct healing effect other than an "Oh shit" button that lasts for 10 seconds every three minutes? No. This does not compare at all. Still, priests were effective healers for parties and raids. Circle of Healing now had a cooldown, but Prayer of Healing was putting out great numbers if you stacked regen to support it.
Then came the mana regeneration nerf. Priests relied on stacked Spirit for killer mana regeneration and even to boost healing output. Druids did to, but they had innervate to help offset the nerf. Pallies regenerate mana through their own tools and through crits. Shammies never used Spirit for much of anything anyways. Mail gear is heavy in mana per 5, and they have very effective water shields for boosting mana pools. Suddenly... my priest had less than half of the regeneration it had back at level 70. Yet my spell costs are double in some cases. Spirit became a joke. Mana issues plagued priests until the other casters helped us out with replenishment. Prayer of Healing became our staple raid healing spell. It could be alternated in a loose cycle with Circle of Healing while Prayer of Mending made the rounds. We were suddenly the bottom of the rung in healing. Meters aren't everything, but they sure are clear in this regard. Our numbers were bad.
Then came the Prayer of Healing nerf and the replenishment nerf with the last patch. I won't even elucidate on how unnecessary or damaging this was.
A priest can still rock the charts and be a valuable raid healer... but we have to work twice has hard as the other healing classes for it. My guild takes a priest on every raid for Fortitude and Divine Spirit. Our main healing team, which I used to lead, is now stocked with paladins and druids instead. I still put up some pretty decent numbers, but a druid or paladin with crap gear can field the same or better if they have even half a clue.
Blizzard has no idea what they are doing anymore with class balance. The recent priest Q&A was a case study in self-contradiction and half-truths.
In the meantime, my paladin is coming along nicely.