One of the ideas that I always find interesting is that, as raid difficulty increases, it becomes more reasonable to expect certain abilities to be available. Like in this tweet from Nathaniel Chapman, an encounter designer for WoW, which talks about the Hand of Protection ability. (As an aside, I very much recommend his personal blog.) The basic idea is simple, and can be extrapolated to other abilities - it's reasonable to expect a Mythic raid to have access to abilities such as Hand of Protection or other class specific abilities that can alter a fight's parameters.
It's this idea of raid difficulty making specific design choices acceptable that I find interesting, at least in the context of class abilities that might otherwise be seen as unbalancing. One specific mention was how Paladins could cheese tank swap mechanics, something that made them invaluable on fights like Heroic Horridon. Imagine that, in a hypothetical Mythic Horridon, the fight was designed for you to cheese tank swap mechanics, or the boss came with a huge AoE damage spell that was spell reflectable, justifying the inclusion of a warrior tank or a DPS warrior with Mass Spell Reflect. The cast could also be spellstolen, so mages would be a valuable addition.
These kinds of mechanics are seen (and rightfully so, in my opinion) as punitive to struggling small raid groups who only have so many combinations of classes and specs. If your raid doesn't have a mage or warrior, for example, then dealing with that hypothetical huge AoE damage attack becomes harder. But for Mythic difficulty, with its iron-clad 20 player limit, you can expect more diversity in raid makeup, and thus can design for it.