Other characters used to be seen as nuisances long before CRZ was around. But the problem is not the levelling speed, it's the fact that a) most key quest monsters weren't multi-tap (until a few days ago) and b) WoW does not favor leveling in group because of silly xp modifiers. There was literally no incentive to level in groups from the start on. Which, incidentally, was one of vanilla's selling points compared to MMOs where leveling had to be done in a group.
That's not absurd extremes, that's how leveling e.g. a fire mage was in vanilla. You literally had to drink every 2 or 3 mobs.
They cast annoying heals or other spells and interrupts can be on CD
Again, my concern is about the time it'll take to level up if you make "enemies in the world to pose threats". This being said, I think that at this point, levels should just be scrapped because either choice (long leveling time or easy mobs) are bad, so we need something else.
I agree, heirlooms need to be redone somehow. Personally, I always was in favor of a straight +xp% buff on your account/realm once you got a max-level character.
Or better yet, allow character to have multiple professions so I don't have to level a bajillon of alts just to supply my main.
Again, this is vanilla leveling experience.