There ain’t a sliding scale, they each are best at their own thang, and not as good as the others at theirs.
There really isn’t. I know Blizzard like to fetishize Jaina, but she is -just- a very powerful Archmage, a swift punch to the jaw will interrupt spellcasting, potentially breaking her jaw. An arrow from five hundred yards away will still one shot kill her if it is a head shot. Just about any standard Human footman could beat the living tar out of Gul’dan, he’s crippled anyway so part of the work is done for you (as a Crip myself I can make that kind of gag:P ) He seriously is not all that and a bag of chips. I mean Warriors can apparently One Shot Fel Reavers when at the top of their game, and even a Priest is capable of driving a sword through plate armoured tanks. Hunters -can- actually summon animals out of thin air, and can kill you before you even know they are there. Know what they call a Demon Hunter or a Death Knight with an arrow in their head?
Just as Dead as anyone else.
The ‘Hero’ classes referred to the playing experience, that you start at a higher level, -not- that they were inherently superior to the other classes, a fact which Blizzard clarified (If you type ‘Hero Class’ into Wowpedia it gives the exact quotes and sources)
The truly powerful will use their powers in ways that are not so obvious, instead of just going all ham mega sorcerer.
“I see you, Tobias Arcwright, the plucky peasant hero with his crossbow, I know of your puny quest for vengeance, what can I say, your village was in the way, I had to subjugate the nose sucking Demon of Archoo, and besides, I am an Arch Warlock, doing nasty things goes with the dribbly candles and skulls on my robes. Stand in sheeplike terror as I monologue evilly and you just let me do nasty stuff, just like Varian Wrynn let our Lord and Saviour Gul’dan do at the Broken Shores. Such magic I will summon, I call forth the nethermost reaches of torment and agony to invoke my spell, a spell of utter horror that will rend your physical form asunder and make spiralling ribbons of your soul, to float on the ether, conscious, suffering, yet unable even to die and know peace, I CAST -”
Toby Arcwright: “Get him Johnny Chimpo!”
A small, yet furious Macacque starts trying to enthusiastically unscrew the Warlock’s head
Warlock “Arrrgh, Whoah is me! Brought low by someone too stupid to know that a Macacque is a monkey, not a Chimp!”
Tobias chews a stalk of wheat “Ayep, but ah’m not the one with Johnny Chimpo eating my face…”
No one class is better, that’s kind of the point. Its all circumstantial.