After much deliberation on which toons to level to 100 and focus on for WoD I finally narrowed my options down to the four I was most interested in: two druids to cover all four specs (obviously I love druids), a rogue, and a monk. My main druid is feral/guardian and an engineer, my favorite class, specs and profession. My second druid is a leather worker which is handy since all my favorite toons wear leather. My rogue is an enchanter/tailor so I can enchant, make bags, and unlock lock boxes. Finally, my monk (previously also an engineer) has taken up alchemy to keep me stocked up with flasks for raiding, and just happens to not have a DPS spec.
At the end of the last expansion I was itching for another heirloom off of Garrosh, so I dropped my monk's DPS spec for a tanking spec so I could be guaranteed a drop by bringing a different toon, and be able to fill my normal role. As it turns out everyone had similar ideas that last raid before the expansion and we never made it to Garrosh, but I got some gear and had fun playing a monk tank for the first time at what was then the max level. Before that I've only tanked on a monk alt which has only made it to level 55.
So long story short when I wandered over to the dark portal to start leveling up, it slipped my mind that I had dropped my DPS spec. I was going to hearth back and address that when it was brought to my attention that tanking damage has been changed and I probably didn't need to have a DPS spec. A combination of curiosity and laziness compelled me to give it a shot, and I am glad I did.
Leveling using a tank spec is actually more efficient than using a DPS spec, at least in case of a monk vs a rogue, but I suspect this is also true for other classes. I think a large contributing factor is that this is an alt swimming in rested experience, and a large advantage of being a tank is steamrolling large groups of mobs. On my rogue I have to be much more careful about which mobs I pull and how many I can juggle at a time, but as a tank I just grab everything I can see, pummel it into the ground, and then see which quest objectives I missed. More times than not for a gather X items or kill X mobs quest I finish it on the first try and then some. It doesn't really tank any more time to AOE down a few more mobs, and they just add up to more experience.
So there is a caveat, leveling using a tank spec requires a change in tactics to play to your strengths. AOEing large groups is possible which allows some multitasking completing multiple quests in the same area, but when single target DPS is required, for rare mobs for example, your DPS will be noticeably lower than a DPS spec and killing that rare will cancel out some of those time savings elsewhere. Despite being a little slower, I've also noticed that I can take out rares and elites of a higher level as a tank; there isn't an enrage timer on them so slow and steady does indeed finish first.
I am enjoying my tank/healer dual spec and how that balances with the rest of my four toons. I've got two tanks, two healers, two melee DPS and one ranged DPS. There just isn't time to play every class/spec, but I'm already eying my hunter and death knight. I'm toying with the idea of putting a little time in on the Horde side this expansion and coincidentally created a Horde hunter and death knight right at the end of Mists. First things first, have to get geared for raid content.