Crippling Wave - Concussion 20% dmg red 20% attk spd red 30% mvt slow
Shouldn't need much explanation here. I see no reason to run multiple spirit generators as a tank; the damage-reduction-runed options on things like Deadly Reach are only self-applied, and I don't see them as remotely worth an ability slot.
Lashing Tail Kick - Scorpion Sting 50% chance 1.5s stun
This seems to be left out of many Monk tanking builds, but strikes me as one of his best tanking abilities. This could lead, with sufficient spirit generation, to chain stunning at least a substantial subset of mobs. Every 5 CW (Crippling Waves) will get you a LTK (Lashing Tail Kick), which means, by my estimation, every 2-3 seconds with gear options, maybe a smidge faster. Even if it's only every 3 seconds, then that constitutes roughly a 25% overall damage reduction to all players on average, and an average root on 25% of nearby mobs.
Cyclone Strike - Implosion
I could conceivably see going to Eye of the Storm for the rune, but I don't expect it will be necessary often enough to require that. This would mostly be to open fights, and to protect people if/when more creatures run in or leave the Monk to go after someone. Most of the spirit dumping would be on LTK. To me, this serves the same purpose that Dashing Strike (or even Tempest Rush) might in the context of a tank: protecting your teammates (though it has no utility as an escape for the Monk, but your tank shouldn't be trying to escape anyway, they should be protecting everyone else.)
Breath of Heaven - Blazing Wrath 15% dmg inc buff
I could see going to Circle of Life rune if you had a hard time surviving, but I think the damage buff will be a worthwhile thing to keep up on everyone; I would probably try to spam BoH (Breath of Heaven) every 15 sec just to keep the buff up on everyone (15% from Blazing Wrath as well as 16% from Guiding Light; that's 31% net damage increase on your party, vs an avg 29% increase in net healing by going to Circle of Life.
Serenity - Tranquility
Should be pretty self-explanatory, but to clarify, since I most-commonly see Peaceful Repose in people's builds: the function of a tank is not only to survive, but to protect everyone else; this rune form means preventing or shedding CCs from your party; that's invaluable. Similarly, Peaceful Reposeis a heal that won't affect them, so people who think it can replace the function of BoH as a tank are being... more than optimistic, imo.
Mantra of Conviction - Intimidation 12% inc dmg debuff 10% dmg red
Any version of Mantra of Evasion (MoE) (except Backlash ) or Mantra of Healing (MoH) (except Circular Breathing) are reasonable options; especially Boon of Protection on MoH, if you are having trouble keeping people alive, as it is one of the only direct/controlled methods of group healing/damage prevention other than BoH. Even so, I think MoC (Mantra of Conviction) with Intimidation will, in fact, constitute the most damage reduction/survivability of any of these, not just provide the damage buff; expansion on this below.
Alternately, I could see using: One with Everything if stacking resistance turns out to be difficult/a problem at higher difficulties, Seize the Initiative or Sixth Sense (whichever turns out to be better given gear/stats/scaling) if general survivability is a problem, or Pacifism if Serenity isn't enough CC countering to keep you safe.
Let's count damage reduction. Personally, I'm uncertain at the moment how things stack: do damage reduction debuffs, or damage buffs, etc, add, or multiply, and does that depend on whether they are from one source or different sources, etc? The two cases here are: 20% damage reduction - CW - Concussion 10% damage reduction - MoC - Intimidation 25% damage reduction - Resolve
which means either 20% + 10% + 25% = 55% damage reduction, or 1 - 0.8 * 0.9 * 0.75 = 46% damage reduction; ~50% either way. Note with respect to MoC w/ Intimidation: this means that the 10% damage reduction provided constitutes roughly a 20% net reduction to incoming damage to all party members, easily beating out the 15% dodge available on MoE (assuming no/low base dodge values) for most teammates. Beyond this, we can include the 20% attack speed slow of CW, as well as the averaged-over-time ~25%-or-better time-spent-stunned of LTK, which means ( 45% or 54%, depending) damage output * 80% attack speed * < 75% action time for anet < 27% or 32.4% damage output from mobs, or> 73% or 67.6% damage reduction (depending on whether the direct damage reduction debuffs stack additively or multiplicatively.) That seems pretty good to me.
On the protection and control side of things, we have the 30% movement speed slow on CW, 50% chance of 1.5s stun on LTK, the 24 or 34 yd draw on Cyclone Strike, the 6202-7442 heal on BoH, and the CC prevention/shedding on Serenity - Tranquility. The Guardian's Path in 25% increased spirit generation mode lends itself to this aspect of tanking, as that is a 25% more stun time (or more, since some spirit goes to Cyclone Strike and BoH, not all to LTK), making it a more-worthwhile take than passives which only reduce damage taken by the Monk, not the party.
As far as damage, we mostly rely on CW's 110% to adjacent mobs and LTK's 200% to adjacent mobs, as Cyclone Strike won't come up many times per fight. However, the majority of the killing power in this build probably comes from the 15% and 16% damage increase buffs on BoH - Blazing Wrath and Guiding Light generated from BoH heals; as both buffs last 15s, and the BoH CD is 15s, I'd expect to spam it every time it is up, so long as most players will benefit from a heal at the time; to this we add the 12% damage increase debuff on MoC, totalling (1 + 15% + 16%) * 1.12 = 46.7% buffed group damage... which is very substantial (certainly greatly-exceeds the damage buff potential of any other class.) In the unlikely case that you are breezing through mobs so easily that damage reduction is not a concern, MoC can be switched over to Overawe, going from Intimidation's 10% damage reduction to Overawe's additional 12% damage increase, and CW can be switched from Concussion's 20% damage reduction to Breaking Wave's 10% damage increase, for a potential total of yeilding 75% damage output * 80% attack speed * < 75% action time for 45% damage reduction (55% damage output from mobs), and (1 + 15% + 16%) * (1 + 24% + 10%) = 75.5% buffed group damage for the party; the near-equivalent of adding 2-3 players.
Dodge and armor are comparable when they can be applied to the whole party, but I would say they are not even comparable when available only to the Monk (this being the case with passives or runes on abilities, though not for mantras.)
Hm, I read Blazing Wrath as also affecting anyone affected by BoH; if it's a self-buff only, I'd probably go for Infused With Light so as to produce more spirit -> more stun spamming. Can anyone confirm exactly what it's effect is?
I don't really see a lot of need for a teleport effect if you have Cyclone Strike; your party members can just stay behind you (or in whatever configuration makes sense/is safe, and you should be able to just pull all the mobs too you without much difficulty, I imagine (certainly, this claim is untested, on my end.) If that assumptions breaks down, I might consider Dashing Strike - Flying Side Kick or Quicksilver (probably not Fists of Thunder) as a gap-closing solution; I'd probably take out Serenity if it came to it, but not happily.
As far as soaking, I can concede that if resists are too hard to come by, or CCs are too much of a problem, One with Everything or Pacifism might become necessary, but I think it will be quite possible to get away without them.
The group I expect to be in will be all-different classes; it had not occurred to me to consider how I would spec a monk if I expected to have two (or more.) As such, this build might be a bit hybrid-y, but in a game like Diablo, I think even a hybrid-y tank will be quite sufficient with a good group, since you can play the game tanklessly anyway. When the concern is overall group killing power per time and survivability, I think these tradeoffs are acceptable.