I just got my Monk to 110 (I think this expansion I've been leveling more characters to max quicker than I've ever done before. I think a huge part of that is the class campaigns, which give me something truly new to do with each alt.)
Anyway, while my DK and DH have both been flirting with going mainspec DPS this expansion (on LFR, I've been going primarily Havoc and Unholy) I've been starting to gravitate back toward tanking on both of them, especially as it's a hell of a lot easier to get into a Mythic dungeon group when you're a tank.
My Monk, while he's never been anywhere near as top-priority as the Paladin or Death Knight (or Demon Hunter, who very quickly jumped up to my #3 spot,) has, like the Paladin, always been a Tank as his mainspec. You really cannot ignore the amazing and, well, hilarious flavor of the Brewmaster tank, so while I've played around with Windwalker a bit, it's really undeniable that he's a Brewmaster through and through (often going around with the Brewmaster title, though right now he's going with Crashin' Thrashin', which seems to fit even better.)
Anyway, having now tanked a decent amount on four classes (I did a round of five timewalkers with my guild on my Warrior as Protection, but that's about it) I thought I'd talk about them. As a big caveat, the Paladin is definitely better geared than the others, though that gap is shrinking now that my DK and DH are both only one gear slot shy of Brokenly Epic, and in terms of average gear level are only one or two points away from the Paladin. The Paladin is definitely ahead in terms of artifact traits, though. The Monk, on the other hand, is far behind, not yet even doing heroic dungeons.
Given that I've been playing this class and spec for about ten years, tanking on a Paladin is literally the main way I experience World of Warcraft. I try to make sure to take him into every instance first, before doing so on other characters (actually, I cheated tonight taking my Demon Hunter into Mythic Neltharion's Lair - the Paladin hasn't done that or Vault of the Wardens Mythic yet.)
Paladins are, as I've usually found, feeling pretty strong. The natural damage-smoothing of the block mechanic couples well with fairly reliable active mitigation and potent self-healing in the form of Light of the Protector. Having Judgment and Avenger's Shield really helps when you need to pick up threat on distant enemies, and I've found that taking the Blessed Hammer talent gives you a great tool to help get snap-threat and cover entire areas with at least a little threat now that Glyph of the Consecrator is no more.
Having to stand in your Consecration to get the full strength of your defensive abilities does reduce your mobility a little bit, but I think that fights are being designed with a little less mobility required now, and your Consecration is large enough that you'll usually be able to find a spot within it that's open. Also, with Consecration's cooldown but not its duration reduced by Haste as well as artifact traits that increase its duration, you can often have two Consecrations up at a time.
The charge-based system for Shield of the Righteous does require a little more forward-thinking now, but unless the boss has a specific ability that requires active mitgation, you can basically just use it on cooldown (though I recommend using Weak Auras or something similar to track the duration of the buff so that you don't needlessly overlap them - though I think there's a little bit of overlap protection built-in, so it might not be that much of a problem.)
I really can't get over how smart it was to make Death Strike the Runic Power spending ability, as previously it always felt like a total waste to use runes on anything else. The Marrowrend/Heart Strike choice keeps you thinking about what to do constantly.
I also love the new Blood Boil, as it's a free and very frequently available AoE burst that also applies your spec's disease. Blood Boil is basically the ideal snap-threat ability, totally untargeted and doing a pretty sizable amount of damage in an area.
Death Knights don't have the best ranged-threat options. You can throw your Death and Decay over to distant enemies (and in fact it's very good for grabbing packs) or you can use Death's Caress. Unfortunately, Death's Caress doesn't really do a lot of up-front damage, and so while it's fine if there an add that none of the DPS is going after yet, it's not going to be all that effective at taking attention away from the Hunter who just pulled it with Barrage. Still, there's always Death Grip, and Gorefiend's Grasp as well.
Survival-wise, you've got to get used to using Vampiric Blood more proactively, and the artifact ability Consumption can be a nice burst heal when you're tanking big packs (on single target it's pretty underwhelming.) Still, with a massive health pool and super-powerful self-healing from Death Strike, Blood's also in a pretty good place for survival.
So here we go from the two least-mobile tanks (Paladins and Death Knights are very much "plant your feet" tanks) to the two most mobile.
While I suspect that Death Knights are the most self-healing-focused tank spec, Demon Hunters can't be too far behind. Soul Cleave, especially when you have a decent number of soul fragments out, draws in a massive amount of healing, and like Death Knight's you're going to see your health bar bounce a bit more than some of the other tanks. For that reason, I wonder if Demon Hunters shouldn't have a bigger health pool. My DH and DK are almost at the same gear level (in fact, I think the only piece either has below 840 the chest piece,) but my DK has about 500,000 more health. Granted, Demon Hunters have Demon Spikes while Death Knights really have to rely almost entirely on Death Strike's healing, but I'll tell you that the heal out of Soul Cleave is usually pretty necessary.
Vengeance is fun, but there are definitely some reliability issues - Shear's chance to grant Soul Shards is very hard to predict, and so while you'll sometimes have three out and your Soul Cleave will totally fill you up, other times you'll be getting "raw" Soul Cleaves that are pretty underwhelming. The artifact ability Soul Carver does really help with this, though the 40 second cooldown means it's in that awkward middle ground between being a cooldown and a rotational ability.
Threat-wise, Throw Glaive is very reliable for snap threat, and of course Infernal Strike is great for that too, assuming that positioning isn't going to need to be too precise. Your Sigils are also pretty great utility spells.
With the loss of Guard and a far less reliable Expel Harm, Monks have really focused on the Stagger mechanic. While before Legion, it was pretty exciting to get Stagger above 50%, now you'll spend probably most of your time with a Stagger percentage of 80%. You no longer get any boost to armor or flat damage reduction (which I think would make any bonus armor items extra-powerful for Monks) so Stagger's the thing you've really got to rely on.
Especially with the talent Black Ox Brew (which resets the charges on your Ironskin/Purifying Brew, restores your energy to 100, and is itself a Brew and thus will have its cooldown reduced by Keg Smash and Tiger Palm) you can maintain very high uptime on your active mitigation, Ironskin Brew. This does make fights like Dargrul or Illysanna Ravencrest pretty trivial (unlike for Death Knights, who have to be very precise in timing to make sure their Blood Shield is up.)
Much like Paladins, Monks have to think carefully about their active mitigation abilities, though again, thanks to Black Ox Brew and the ability to actively regenerate your charges, it really only becomes a challenge when you're taking enough damage to start weaving in those Purifying Brews. I've found that from 100 through 109, it was rare that I had enough staggered damage to need a Purifying Brew. Upon hitting the cap, though, running through Halls of Valor again for a class quest saw my stagger bar going yellow and red pretty frequently.
That said, in those cases it was typically my stagger bar that got alarming, and rarely my actual health bar.
So survival is ok, though it'll require a lot more reactive maintenance than it did back when Guard absorbed about a billion damage.
Threat is a little trickier. For enemies in melee range, Keg Smash continues to do massive damage and will keep things glued to you. You also have Breath of Fire as a real part of the rotation, and not just something that you're wasting Chi on. However, while the range on Keg Smash has gotten bigger, grabbing far-away adds is not going to be as easy as it used to, now that Dizzying Haze is gone. You can actually use the artifact ability Flaming Keg to do a big burst of damage at a decent range, but given that it's really more of a defensive cooldown and is also only available once every minute and fifteen seconds, you'll probably want to have other options. There is Crackling Jade Lightning, but the damage it does is pretty pathetic.
In fact, there's really not any AoE ability that doesn't have a bit of a cooldown - Keg Smash and Breath of Fire are both AoE, but won't necessarily be ready when you want them. So there's basically tab-targeting if you get adds streaming in at an inopportune time.
Tanks to Come:
My Druid will probably go Guardian this expansion if I'm not once again torn between that and Balance. My Warrior will probably stick with Arms, but if I decide I really want to take him through Mythic dungeons, I might be inspired to have him go Protection. Hopefully some day my Shaman or Rogue will be able to find Mythic groups that want them (my Shaman in particular is capable of pretty insane DPS these days) but for now I'm pretty happy playing all these tanks.