I've been thinking about class balance lately as I look at my own raid team and I thought back to my raiding career over the last couple of years. This is going to be a bit long and rambling, but hopefully productive. :-)
When I started raiding, I was very good friends with a Holy Paladin who healed my bear through all the TBC heroics, Kara and Z'A. A Holy Paladin/Bear combo was pretty beastly in TBC raiding. Bears had massive armor and health (more than any other class) and Holy Paladins could keep spot healing them all day long. Back then, we were the only class that would eat Crushing Blows (unless one of the shield tanks missed keeping a buff up) and the Paladin would just land a big heal to cover it. We could keep a boss going till enrage fairly easily. During this time my tanking partners rotated a lot but I raided a lot with another paladin tank (who were strictly AoE or main tanks and made terrible off-tanks just due to the amount of agro that Holy Shield generated.)
In Wrath I swapped to my Paladin, and we ran all of T7 with two-healers (Resto Druid and Holy Priest) who were both capable of keeping the tanks and raid up through some hectic damage. Running with a Holy Priest was great, and I didn't really notice a difference, but running with a Resto Druid really required an adjustment on my part - I couldn't get to seeing my health deplete so much before being stabilized and restored by HoTs. I would frequently freak out before I realized this was just how it worked with HoTs.
But one of the problems I started running into was a question of resources - as a bear tank, I never ever had to worry about rage unless I was in a really piddly instance where I took no damage. Generally speaking, I had enough rage to cover my needs at any given moment. But as a Paladin, I had to be continuously getting attacked or taking damage in order to generate mana or I'd OOM like crazy. Blessing of Sanctuary was also useless if I was second on agro and this caused a lot of issues especially in AoE situations that I didn't start
I also became aware of the differences between Bears and shield tanks. I didn't have that massive health advantage anymore, nor the armor, but we technically didn't need it - there were no more crushing blows, but Wrath added burst damage to compensate so all tanks were taking equivalent blows. Saphiron's breath for example, was a good early indicator of things to come, and tanks with huge mitigation cool-downs became king. Nobody was doing Saphiron with drakes up without a DK tank just because AMS was beastly on that fight. For a little while, it really did create nitches for progression raids where certain classes were just really good at certain fights.
Ulduar continued this to a certain degree with Iron Council where Paladin tanks could eat Steelbreaker's Fusion Punch a lot better than anyone else because we could self-cleanse almost before it had a chance to tick (I remember I would begin the cleanse when I saw the cast had less than a second left and almost never actually saw the debuff land or tick.)
At the end of T7 early T8 I actually had to begin taking account of my threat for the first time. Usually MD was a big boost and in TBC we had BoS and so forth, and DPS waited a long time before (let mangle cap) but as we got to Ulda and the AoE mindset kicked in, I had to adjust my gearing strategies to get to hit cap and start stacking expertise.
This was when I started to become addicted to threat sets but we'll talk about that later.
I started raiding late into Ulduar and by the time we started work on hard modes, ToC was already out. T9 took the idea of burst damage and made it just ridiculous. We had a fair amount of gear from Ulduar when we started raiding but 2-healing was just about impossible at that early stage, everyone was running with three healers all the time by then. Most of the fights were not too complicated, but heroic ToC really demonstrated what was to come. Northrend beasts became a massive cock-block for many people because of the amount of damage that Magnataur would do to tanks. Mitigation gave way to stamina at this point, and tanks ballooned with massive health pools.
Death Knights and Druids got the bulk of that buff, as they lacked mitigation and couldn't depend on block so generally their health was much bigger than shield-tanks. But shield tanks generally speaking had a more predictable level of damage intake and we still remained competitive - I didn't have to step aside or off-spec at any point and we managed to get up to Heroic Anub while still in T9 gear.
By now, shield tanks had been used to block-cap and massive mitigation and the only option for Blizzard to continue building raid tiers without having every boss do magical crushing blows all the time was to nerf the mitigation - thus the ICC debuff. To be honest, I remember the first wing being a complete push-over, I don't think we had any trouble at any point. It wasn't until we started hitting Blood Queen and Putricide and Sindragosa that we realized it was literally all about stamina at that point. Every single enchant, gem and bonus that could be used for stamina was used for it. I remember in tens having around 60 thousand health with raid buffs and it felt huge. You could take a couple of massive hits and not die, give the healers time to catch up to you.
That's also why a lot of bleeding edge raids used bear tanks - they understood just how effective that massive health pool was - mitigation didn't matter, and you were going to get hit, really really hard. But with infinite mana, healing through it was no problem. To be honest, gearing this tier was really dull and capped really quickly.
Having that giant healthpool was such a huge asset on Lich King and on certain heroic fights (especially Heroic versions of Sindragosa, BQL, Saurfang, Putricide...) and when we came into Cata, I was ready for more of the same.
And then came patch 4.0.3 and we received, I think, the first half of a good job of changing things around so Stamina isn't king anymore. We worry about mastery these days, and quite a bit, mitigation is a good thing thanks to healer mana being a real issue. This made gearing up have meaningful choices and on certain fights it became a question of using more stamina or mitigation. For example, if you're using a stamina set on Chimeron, there's something profoundly wrong.
But along with Vengeance came the biggest change to threat where threat stats didn't matter. You don't need to be hit capped or expertise soft-capped anymore. It just doesn't pay-off except maybe for missed attacks costing an ability or failing to restore a resource (Holy Power, Rage, etc.) And this is something I miss.
Once we were in Heroics in ICC, I was used to providing a significant amount of damage to the raid. I was always hit-capped, I was deep past the soft-cap on expertise, and in a full threat set with heroic gear and an expertise trinket, I was hitting close to the hard-cap. This was a lot of fun, and I'm sure it would still pay-off today if I had a set like that, for heroics or something, but at least in Raids it's just not necessary. You wind up taking more damage which drains healer mana and that's too valuable a resource.
Now, you might say that this is Blizzard's way of forcing tanks to actually spec and gear for tanking and not damage, but I would venture to disagree - the total lack of Threat issues makes tanks essentially damage soakers, and I find that it looses a bit of the dynamic fight between DPS and Tanks, and the pride I used to take from seeing my threat lead grow and pull past the closest DPS or something even the OT.
So... where are we now? In a better place as far as tanking goes? I think so. The reduced spike-damage, emphasis on mitigation, limited healer mana and fights that require movement, cool-downs and synchronized execution are all great - but we lost threat along the way. Would that factor cause things to become too complicated, as people seem to think the current tier is too hard as it is?
Maybe, but I tend to disagree. I think the current tier is either just a hair-tad over tuned or just about right. If you threw threat in as an issue, it might still hold up, so long as tanks paid attention to their threat stats. And I know every time I reforge Hit or Expertise into Mastery or Dodge, a part of me protests.
(PS - I also raided as a Blood DK for a while in Wrath, but only in heroics and it was very trivial to build a full-threat set with massive AoE and I never noticed an issue. Death-striking for self-heals and access to the number of CDs I had as a Blood Tank made me dearly wish I could raid with him, but I never really got a chance, and just as I got him up to T10, my guild had more or less closed the door on ICC after getting the drakes so I never really got a chance to try raid tanking with a third class. I'd also like to try warrior tanking as I've only ever DPSed on him except for a brief stint while leveling back in TBC days, but who knows. I'm hoping I get the chance in Cataclysm to try all the other tank classes. )