Where we were
Back to Vengeance briefly -- when the mechanic was first introduced, it was billed as a response to a problem that had arisen with the unexpected item level inflation of Wrath. By the end of the expansion, DPS throughput began to truly test many tanks that had spent the previous tiers of the expansion stacking more and more survivability while DPS were improving their damage. The threat ceiling loomed close in raids for many players.
Vengeance, which allowed tanks to scale their threat output in relation to the damage they were taking (which scaled tier to tier), generally solved that problem -- for good or ill, as some would argue. The buff to threat modifiers mid-way through Cataclysm then outright removed any further worries of threat from a tank's mind, outside of sticky threat swaps.
While a success with its effect on the raiding sphere, the footprint of Vengeance reached far wider than just the boundaries of any raid. As anyone who leveled protection in Wrath and then Cataclysm can attest, those were two very different experiences, thanks to Vengeance. In Wrath, where we could do 30-40% of the average damage of a DPS player, we could eventually kill a mob in a one-on-one fight before either us or the mob died of old age. Arguably it was the golden age of tank (solo) single-target damage. In Cataclysm, with the yoke of Vengeance firmly on our necks and tanks only able to do about 20% of the damage a DPS could, we were not so lucky.
This extended to being an issue for many tanks when it came to doing dailies. In Wrath, I always made a point of doing dailies in my main, protection spec. I was way more familiar (and comfortable) with the playstyle, and I appreciated the extra survivability that being protection offered. In Cataclysm, unless I pulled a huge crowd of mobs, there was just no way it made sense to do dailies as anything other than retribution. It would have taken me three times as long to clear the same quests in protection spec.
A thin line to walk
There is a balance to be had with regards to tank damage output, though we do not currently have it. At one extreme, you'd have battlegrounds full of pseudo-tanks running around shield-slamming clothies to death and guffawing at rogues that furiously attempt to sneak their tiny daggers past layers and layers of plate and hit points. At the other extreme, you have what we have now: where you should really only spec tank for group, PvE content (and maybe some very limited PvP situations) and for anything else you switch to your DPS offspec.
Think of all the new content that has been added in this expansion that you'd be crazy to try to run in your preferred protection spec. If you were doing a Troves of the Thunder King run, you'd get bogged down by the very first troll you accidentally pulled. Sure, you wouldn't die, even if you stood in an alarm circle and got multiple mobs on you -- but you'd waste way too much time trying to kill them all. Time is of the essence, and none is available to waste.
The Brawler's Guild is similarly a terrible idea to try to do as a tank. Considering the enrages, both on the mob and the crowd of real players that you would be holding up by taking forever to kill your opponent; damage capability is of the essence. And while as a tank you'd ostensibly have a huge advantage (game-breaking perhaps), if the arena nukes you for taking too long to kill something (and it will), that advantage ultimately isn't worth squat.
My question is: is the current situation the best solution for the game? Isn't there a happy medium between the two extremes -- of god-tanks and our current weak-noodle tanks that require Vengeance to kill anything in a reasonable amount of time?
I understand why tank damage output is as low as it is right now. It would not be healthy for the game, for multiple obvious reasons, if being a tank meant that you had a game-breaking advantage over another DPS player in certain kinds of content.
Restoring the balance
I think the current situation has created some quality of life -- and fun -- issues for many players that style themselves as main-spec tanks. Personally, I love being a tank. I love running around with a shield strapped to my arm and the confidence that I am a bulwark against my foes. It is not particularly fun that I have to shuck off that play style anytime I want to do anything in WoW that doesn't involve 9 or 24 other people and computer-controlled enemies.
For example, I would love it if there was the possibility of doing the Brawler's Guild as a tank, with some kind of switch being flipped that made the opponent have less health but do more damage. The primary test could be how well you can perform your active mitigation rotation, much like how a DPS is tasked with performing their damage rotation, all while avoiding the same pitfalls and obstacles of each fight. It would go a long way to make me feel more relevant as a main-spec tank in the wider world of the game.
Nonetheless, all that said, I don't believe that Vengeance is the blame for our present situation (to bring it back to the spark that set this diatribe off). It is but one of several knobs available to the devs that tune tank damage.
Instead, I think the problem is that the devs have decided to play it safe and keep tank damage far too low at the baseline, and use Vengeance to compensate so that there are no major raiding ramifications. This may keep things on the level at the macro level, but for a specific subset of players (anyone who considers themselves first and foremost a tank) there are some very annoying consequences. I really don't think it needs to be this way, we can do better than this.
The Light and How to Swing It shows paladin tanks how to combat the Sha in the strange new land of Pandaria. Try out the new control gearing strategy, learn how to make the most of the new active mitigation system on your tankadin, and check out how to deck out your fresh 90 tank to get ready for any raids!