And I'm not making that argument either jeesh. My argument is, that when the content is exactly the same, regardless of class/faction then I have 0 reason, to level another class, besides simply getting X class to 70, for the sake of doing do. But since you are so stuck on the concept of gameplay, then I'll humor you. No, the classes simly aren't different or varied enough in WoW, to make the gameplay seem like "major content". A ranged class feels like a ranged class, a melee class feels like a melee class. Some classes are easier to level due to specific elements of their toolkit, and some harder, but all in all it's pretty much the same.
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Nor did I call Covenants campaigns class campaigns. But there were still 4 of them, which - until later in SL - meant that if I wanted to play those; I'd have to level four characters. Which usually means one main and three alts.