Hardest:
Tank: BloodDK - Mainly because their primary form of AM is Death Strike, which requires them to time it properly to get maximum use out of it. If you don’t use DS properly, you feel the burn. DK is all about timing your CDs properly. Many other tanks don’t have quite the same time-sensitive application of their CDs. For example during a pack start there is rarely a bad time to use Shield Block or Ignore Pain, you simple use them if you can. With DS it’s about timing the hit so it comes after bursts of damage else you get little mileage out of it. DKs also have a harder time kiting than other tanks do. So even is warrior is more challenging in terms of finding the content more difficult (due to scaling largely) I don’t think prot warrior is “harder to play” it just gets way less throughput in how it plays.
Healer: HPala. Largely because HPala has the unique distinction of having to manage two resources where other healers only have one. Holy Power is important for spot healing and AOE healing so managing it is key to success so you have to engage in the sport of generating adequate holy power in preparation for bursts of damage or aoe damage, which means choosing to heal a beacon target directly for example because you know it will net you the 3 Holy Power required to spot heal the other lower HP guy so they’re both topped off, whereas if you healed the other guy directly, he’ll be fine but the beacon target will either have to wait for a cast or a holy shock for example.
Throw in stuff like a very big utility toolkit which can make for interesting plays, a positional requirement on the mastery (which begs you to go into melee often and move around) as well as a potential to deal some fair burst damage and I believe Holy has a pretty complex kit which is easy to get some use out of, but when it comes to utilising it properly and fully, it takes some work comparatively. I’d say disc priest is close though due to the manner of their healing.
DPS: I haven’t played enough DPS In SL to make a great judgement here but i’d probably say based on my experience sub rogue still continues to be a quite demanding spec in terms of keeping track of SD charges, Rupture uptime, SLice and Dice, having a pseudo-RNG element on CP generation and having to plan their damage largely around opportune burst windows to function. Their AOE is also fairly convoluted having to maintain rupture and find weakness on as many targets as they can to truly realise their potential. SPamming Shrunken Storm is not enough. As said there may be a more complex spec however but I haven’t played enough of them to say otherwise.
Easiest:
Tank - Druid seems to be to be as straightforward as it largely always has been, with the highest amount of skill being making sure you keep Ironfur rolling properly. Beyond that it seems to be largely “press for threat/rage” and having a healing cd when they need it. I’m sure guardian druids can make some really big plays, but it seems to me the “bread and butter” of their tanking isn’t particularly complex and boils down to generate rage using a couple of skills and then try to keep one AM ability up for as long as possible. Other tanks atleast have more forms of AM to consider (warriors have IP and Block, Pala has WoG, LotP and SoTR, DK has DS which is singular but more complex in application, Veng has DS, Brand, Cleave and Monk managing stagger optimally requires at least an element of timing druidlacks)
Healer: If i’m looking at the basic healing toolkit I would say resto shaman, as their toolkit is remarkably straightforward at the core. Quick heal, slow heal, aoe heal and spot heal. No frills, heals do exactly what they say on the tin. Talents can make it more complex when you involve stuff like Unleash Life or Cloudburst but at the foundations, the kit is very simple and can be played in a simple fashion if you want. They do have a lot of utility however which increases their skillcap.
If i’m looking in a broader fashion i’d say Holy Priest, as they lack utility to juggle. Their complexity largely only comes from managing Holy Words and picking “the right heal” given they have ,many to choose from.
So for clarity’s sake if someone was asking “what’s the simplest healer to just heal a simple dungeon, no frills” i’d say Resto shaman. If they were looking to ask the same question for content that demands a little bit more and they were a bit more curious about wider kit (beyond just basic bread and butter healing) i’d say Holy Priest is simpler and a better starting point.
DPS: Beast Mastery Hunter at the core for me is still the simplest spec. Not a huge amount has changed since BFA and the playstyle is still at core “spend focus on KC when poss, give Frenzy up, dump focus on CS/MS depending on targets”. Their resource generation is almost entirely passive (even BS just increases the perodic gain) and they have the ease of being able to move without compromising their DPS and the benefits of ranged play. The common contender for their spot: Havoc, has to deal at least with melee-unfriendly stuff which makes dealing their damage a bit more difficult, especially as their big setup ability Eye Beam is telegraphed and can leave you in an odd spot if not timed correctly. Havoc also has total control over their resource generation which I think is more challenging than having it passively (unless you choose Demon Blades of course) because it means you have to dedicate time to generating resource and planning for when you need it in line with CDs rather than simply waiting and budgeting it as BM does.
I also think Havoc has the potential to be more complex through talents (Momentum build) which changes the play quite a bit, whilst BM, at the core, always revolves around the same playstyle of “maintain frenzy, don’t overcap focus, KC on CD” no matter how you build it largely. You may gain one or two new abilities that you can use to spend or generate focus, but the fundementals remain the same which are fairly simple. Havoc Momentum build requires a new skillshot based play to be realised fully and doesn’t play like an Eye Beam build at all as Eye Beam occupies a very different place in both builds (for an EB build, it’s a Fury generator and burst setup, for Momentum it’s simply a Fury dump)