While "hard" is a very subjective term, we went with those specs that are not so run-of-the-mill. Two of the options are Paladin and Shaman, two classes are faction locked, and then there are Druids — the class that can fill all of the 4 core roles in World of Warcraft with an albeit mixed level of success.
For players who say "how high?" when asked to jump.
Druid
"The earth keeps many secrets; it is our duty to ensure that those must remain a secret for the good of all stays as such." - Hamuul Runetotem
When we put Druid into the "Hard" group, we did so with the * that while playing the true hybrid spec brings a lot of gameplay elements to explore, it comes with a major caveat. Like most classes that can heal in Classic, you will find a lot of groups asking you to fill that role. Balance and Feral (Cat) are both valid specs and have a lot of use in solo content, with Feral having slightly more use in PvP. The class tank spec, Feral (Bear), is nothing like it is at any point after Classic.
Great healer
Travel form is handy between level 20-40
Amazing flag runner in PvP
Great utility
Stealth via Cat form
Innervate is a key talent for getting you into a raid team
Decent levelling speed
Battle rez
Tank spec not viable
Probably stuck as a healer
Boomkin & Cat are fine, but other classes do the job better
Unlike in BfA, you can't stack Druid heals
No revive outside of the battle rez
Is the Druid just another healer? No. Will you get asked to heal a lot anyway? Yes. That shouldn't stop you from living out your best life as a Druid of the claw or being the best chicken caster you can. The Druid also has a number of other pros to consider — especially when it comes to levelling and solo play — that once mastered, should outweigh the cons. With the Mark of the Wild buff and a unique heal over time (HoT) style thanks to spells like Rejuvenation and Regrowth, you'll always be popular, but with limited space due to buff stacking.
Shaman
"It is your duty to aid the Horde and to defend our way of life. But it is also your duty to know when quarter and compassion should be given to friend and foe alike." - Thrall
The shaman is one of the two faction-specific classes — that's right, Alliance, no Shamans for you. The Shaman is a pretty solid class when it comes to Classic. You'll be a good healer, and Enhancement spec will have roughly 1-3 raid slots, which means you can play the spec in PvE — just be prepared for some competition. Elemental is the weaker of the specs — while good in PvP (with good gear), it suffers from the same problem a lot of casters specs have: Mages do your job better.
Totems
Ghost Wolf for travel pre-40
Windfury can do insane burst
Good raid utility
Decent healer
Healing is only viable raid option
Lots of raid competition for Enhancement spec
Elemental is only good in PvP with good gear
Horde only
One of the key strengths of the Shaman is their faction uniqueness. Alliance will fear you in PvP as you kite them around with Frost Shock, deal huge burst with Windfurry Totem and Ghost Wolf away with their flag. Failing that, you can just heal and be the most popular Shaman in the Horde.
Paladin
"Steel your heart and your soul will shine brighter than a thousand suns. The enemy will falter at the sight of you. They will fall as the light of righteousness envelops them" - Tirion Fordring
Like the Shaman, the Paladin is faction-unique, this time for the Alliance. With the light by their side and a Divine Shield on their back, the Paladin is the shining light of the Alliance. Providing you like the idea of being a plate (dress) wearing Holy knight, the Paladin might just be the class for you. Don't expect to do much DPS in raiding, though you might find some success as a tank.
Free mount at 40
Good utility
Great at 1v1 in PvP
Great healing spec
Retribution is great for solo play at level 60
Holy is the only viable raid spec
While Ret has good DPS, it causes high threat
Alliance only
Levelling can be arduous
In PvP Holy is very susceptible to Counterspell
The Paladin really is the plate-wearing priest — it has some amazing healing throughput, adds in some tankiness and some outstanding raid utility with buffs like Blessing of Might and Blessing of Wisdom, throws in a little spice with your own personal bubble (Divine Shield), and gives you a "free" mount at level 40 with Summon Warhorse. Levelling can be slow, but the pay-off is worth it in the end.
