One of the accepted facts of raiding life used to be that the main tank was the guild's gearing priority. As Adam Holisky's observed, "Everything that happens in the raid eventually makes it back to the tank." Healers undergeared? You're screwed. DPS incompetent or just badly grouped? Buh-bye. Random number generator wreaking all manner of havoc on healer crits and boss parries? Thar be the graveyard. A truly cynical mind would opine that the tank should be as well-geared as possible if only because it makes it easier for the raid to forget that person existed as anything other than a rapidly-advancing line on the Omen screen that: a). always stayed above their own, and b). never died. There are enough random variables while the raid's learning a new boss that the tank needs to be eliminated as one, and in vanilla WoW that was certainly the goal. Raid and offtank damage on most encounters hadn't scaled to the point where you could make a compelling argument in favor of gear equilibrium across your tanking roster. What was the point of something like that when 95% of the damage in a fight was going to be absorbed by a single person?
For the purpose of this discussion, we might as well attempt to define what a main tank actually is (you may, of course, disagree with these definitions, but I think they're a pretty accurate summary of the WoW community's perception of tanking roles):
MAIN TANK: A player with excellent raid attendance playing a tanking class specced for that purpose, competitively geared for progression content, and who can realistically expect to tank nearly all boss encounters and/or the more difficult portions thereof.
Sound good? It certainly covers just about everyone who actually had to spec Protection pre-BC. How about this:
OFFTANK: A player with average-to-excellent raid attendance playing a tanking class with tanking gear adequate to support performance in that role. Depending on class, being specced into tanking may or may not be required (e.g. Paladin and Druid offtanks who are not specced Protection or Feral respectively are significantly less useful as tanks than DPS Warriors), but he/she can realistically expect to tank trash and, if necessary, some boss encounters requiring add tanks.
The second is where things start to get difficult in the post-BC world, because the line separating Main Tank and Offtank got increasingly blurry. It didn't get this way because multiple tanks stopped being important; it got that way because Blizzard's raid/encounter design demanded more people specced and geared to do a Main Tank job, even as the developers downsized raids from 40 to 25 players. If you didn't come as Protection or Feral and you didn't have gear within shouting distance of the Main Tank's, you had no real business offtanking Gruul, or rotating taunts on Al'ar, or juggling threat on Gurtogg.
The old paradigm of the DPS Warrior as offtank was the first casualty of both the shrunken raid and rapidly-scaling boss and raid damage. You can still take a DPS Warrior with great tanking gear along on a 25-man raid for trash (ours certainly did), but you'd have to be insane to expect them to do any real tanking on a boss encounter. It would be a pretty bad use of the raid's resources to "lose" a DPS while staring down a boss with an enrage timer, but even if your healers could keep the player up, the Warrior didn't have a prayer of holding aggro. Remember Bob and Larry, our cute little angels of mitigation and threat production? Bob might have been grumpy but OK about something like that, but Larry's over in the corner drinking himself into a coma.
So Warriors remained the premier raid boss tanks, but their share of the raiding pie shrank badly, mostly in the form of losing their previous role as both default MT and default OT. If you didn't need them to tank on any given fight, they could respec and...uh...compete for DPS slots that the raid didn't have any problems filling anyway (a situation that was in no way helped by Fury's awful itemization in early BC raids). Paladins became virtually required for the AoE tanking situations that popped up like daisies between Karazhan and Sunwell and had superior threat on demon or undead bosses, but they had to be Protection to guarantee both effectiveness and survivability. Ever tried tanking Hyjal trash as Holy? Yeah, I didn't think so. And Druids...well, Druids did everything else, but they had to be Feral. I've tanked raid content once before as a non-Feral Druid. Note operative use of the qualifier "once." And whatever nonsense you might have heard elsewhere, Druids had to have a full raid-tanking set chock-a-block with defense on whatever slot they could manage in order to keep themselves from being annihilated by crits. What other "melee DPS" do you see rocking Slikk's Cloak of Placation?
So, in BC raiding, you still need a Main Tank. Shocker, I know.
But you also need another Main Tank.
And...another Main Tank.
With a look at Wowwiki and Bosskillers, I feel obliged to note that on some fights you'll need four.
Maiden of Virtue
Opera (Big Bad Wolf)
Leotheras the Blind*
Opera (Wizard of Oz/Romulo and Julianne)
Gruul the Dragonkiller
Three or more tank fights:
High King Maulgar
Hydross the Unstable
The Lurker Below
Fathom Lord Karathress
High Astromancer Solarian (sort of)
Shade of Akama
Reliquary of Souls***
Mother Shahraz (take a good book if you're one of the soak tanks)
*Leo is technically a two-tank fight, but pffffftt......like Warlocks count.
**You can technically get away with two tanks on Anetheron and Azgalor, but Murphy's Law guarantees that: a). on Anetheron your DPS won't kill the infernals fast enough for a single add-tank to be free every time another one spawns, and b). on Azgalor your sole add-tank will be the first person to get Doom once the raid's exhausted its battle-rezzes and soulstones. You'll have at least three tanks in Hyjal anyway, so you might as well use them.
**Yes, Reliquary is bizarre, and you might conceivably manage it with only one very well-geared Protection Warrior, but that also means depending on the guy who spews constructs all over the raid to take his turn tanking on phase one. He doesn't want to do that. Your raid leader doesn't want to do that. And you don't want to do that either.
Notice something odd about these lists? The farther you go in raid content, the more likely it is that any given fight will 100%, non-negotiably require at least two tanks with a roughly equivalent level of gear in order to maximize the raid's chances of success. Given the array of such fights presently in BC, I would argue that current raid mechanics actively discourage guilds from gearing a single tank at everyone else's expense, i.e. in essence following the "main tank" model. Two badly-equipped tanks competing with a well-equipped tank for threat on Void Reaver is usually a fracas, and it's not going to get any better from there. You don't need a Main Tank And Some Useful Lackeys; what you need is a tanking team prepared to look out for each other, pass gear when needed, and always keep an eye on what upcoming fights will demand from the corps. The Main Tank job hasn't precisely disappeared; it's just that the number of people with its responsibilities has multiplied as the raid's dependence on a single person has decreased.
I'd additionally argue that the disappearance (or at least crippling) of the classic "Main Tank" element from raiding guilds today is a net benefit. In the event that your usual tank can't make it or quits, you're not tasked with shoving an underprepared offtank into the raid; provided that people have taken the team approach seriously, you simply rotate in another tank who's already familiar with the job and has the gear to do it. It shouldn't have to be a nightmare. And, while most tanks won't want to admit it, this also acts as a check against tanks' power and ego; someone else can do your job. At the end of the day, everyone in the raid is (and has to be) replaceable, tanks are no different, and I dislike seeing people act as if they're not.
Will this change in Wrath? I doubt Blizzard would introduce a new tanking class, or strengthen the existing tanking specs in the game, without an eye toward ensuring that raid success is not dependent on a single person, so I'm betting not. Look out for your tanking brothers and sisters, people, and keep an eye on those upstart Death Knights.
The Main Tank is dead! Long live the Main Tanks!