From everything I've read, all classes are viable as tanks now. The most 'difficult' one being DKs and even that's supposedly easy. I've had no real trouble with my prot warrior even when I'm teaming with much better geared DPS
LOL DK Tanking is so ez-mode it's not even funny.
The only tank spec I haven't played at all is bear tanking... Of all the others, I'd say Monk is the hardest to play because you have more resources to track.
Bears/Warriors have rage and a few CDs but nothing major (makes tanking pretty brainless, and you always have a choice of which mitigation to go after, absorb all or block physical, which is very good (depending on the situation you want different mitigation).
Paladins have it pretty simple - build HoPo, spend HoPo. Their HoPo builder kinda sucks as it's different for single target and AoE - you can typically use the AoE version all the time, but you lose on DPS. They have 2 mitigation tools: either a 3-second damage reduction or a heal (both of which are good).
DKs are stupidly simple, really. Apply diseases *once*, Blood Boil when it procs (or when you need to spread diseases), then Death Strike on cooldown, use blood runes on Heart Strike (which is also your cleave, how convenient) and burn Runic on Rune Strike. Major point of mitigation is through Death Strike, which heals and places a physical absorb shield on you. Really simple, insane AoE damage/threat due to vengeance being crazy. While you do have 2 resources to track, it's pretty simple really - Death Strike when you can, Heart Strike on 3 enemies or less, Blood Boil on 4 or more, Rune Strike when your Runic is near max.
Monks are a bit different. You have 2 resources to track - Energy and Chi. It works a bit like a Rogue - some abilities use Energy and generate Chi while others use Chi. You have 2 mitigation tools: Shuffle and Guard. Guard is a damage absorption shield that apparently always disappears instantly (I'd have to check, I don't have a 90 Monk), while Shuffe is a buff you must constantly reapply through some abilities that cost Chi.
What I find difficult with a Monk is that you have 3 Chi Builders (Keg Smash, which is on CD and generates 2 Chi, is also your cleave and applies an important debuff, then Expel Harm, which is a mitigation tool as it heals you, deals damage and generates Chi and Jab which just generates Chi), 2 of which are on cooldowns, then you have 2 ways to spend Chi (Blackout Kick to give yourself Shuffle and Guard to put the Shield up), then you have a filler attack (Tiger Palm) that you need to use before using Guard. That's the single target abilities alone.
Then when you get into AoE, you have to apply Keg Smash to have the debuff that's required for Breath of Fire to leave a DoT (or throw kegs, which is *another* button). You also have to keep Shuffle and Guard.
Then you have to deal with Purifying Brew to clear your Stagger, use Elusive Brew to give yourself more avoidance...
All in all, let's see how many buttons you have to regularly hit.
1- Keg Smash
2- Expel Harm
3- Jab
4- Tiger Palm
5- Blackout Kick
6- Guard
7- Spinning Crane Kick
8- Breath of Fire
9- Dizzying Haze
10- Purifying Brew
11- Provoke (the Monk Taunt)
All the while managing your energy, your Chi generation, etc.
Compare that to a DK, where the buttons you'll hit are
1- Outbreak (you'll rarely need to use regular diseases and everything can easily be macroed together)
2- Death Strike
3- Heart Strike
4- Blood Boil
5- Rune Strike
6- Taunt
7- Death Grip
8- Death and Decay
8 abilities vs 11. The way I set up my stuff, I use 1-4 and alt 1-4 as my buttons and I'm all set, so I can easily use 8 abilities. More than 8 and it becomes a bit harder, but then you have to manage more stuff.