I think this is a very interesting question/topic for multiple reasons.
First and most obvious, we have the various classes in the game. Every class (ideally) has a unique set of spells and abilities inherent to their particular archetypes, but I feel it’s a difficult exercise to really compare their power levels “in lore,” so to speak. In the context of roleplay, then, we can consider how our characters relate (or don’t relate) to the particular class archetype they embody, be it a Paladin or Warrior or what have you, and also in relation to everyone else.
Using Karnum here as an example, he’s a Death Knight. In lore, the “third generation,” that is to say the Death Knights of Acherus, are powerful in their own right, but are viewed more as disposable shocktroopers rather than the undead generals of the Third War. However, ICly, Karnum is not from Acherus. He was active during the Third War, and as such, it can be argued that he has superior ‘raise dead’ and ‘command undead’ abilities while also lacking the inherent ‘bloodlust’ weakness of his Acherus counterparts, relating back to the Death Knights of the Third War being the ‘undead general’ archetype I mentioned. Granted, the Unholy Spec now is a bit more in line with the Warcraft 3 hero unit, with Army of the Dead specifically giving the impression of commanding an army of undead minions, but this is after the fact, at least for me. Then we have the situation of him possessing a distinct runeblade that is specific to him, with a direct link to the Lich King, all in the Warcraft 3 fashion. Which, of course, if handled improperly in terms of storytelling can stray right into ominous lore-bending territory, nevermind the potential for “special snowflake syndrome.”
All that being said, there aren’t many opportunities for such magic to even arise in roleplay, and even then I don’t really go for it, at least without prior approval of the people I’m roleplaying with.
On a semi-related note, I find that not everything that exists in lore is feasible in terms of “balance” in roleplay, such as Saronite Plate doing its old-god corruption thing. I suppose the one hand, I enjoy the notion of certain characters being inherently stronger than others, especially if given proper context. But on the other, when you start thinking in such relative terms, I feel the roleplay can be muddled and bogged-down by power levels and such.