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.
Since this is of particular importance in the coming months, it directly impacts how raids that attempt mythic difficulty are going to go about it. For one thing, Mythic raiding is almost certain to demand a more diverse raid structure from those who make the attempt. You want that class with the unique ability that you currently don't see a need for when you hit specific boss who requires it - and since there's no way to know going in whether or not Mind Control will end up being useful (turns out third boss on Mythic difficulty summons an add that can be MC'd and which does huge damage to the boss when so MC'd) your raid will need to be thinking ahead.
Likewise, if raid design feels unconstrained enough to expect these abilites, and you don't have them... well, things are going to be hard. Heroic Thok without paladins hard. Since there will be four raid difficulties in Warlords (LFR, normal, heroic and mythic) you can more easily justify making Mythic, with its 20 player requirement, more stringent in terms of these kinds of design choices. It's acceptable to say "If you want to defeat this boss, get a warlock, because Metamorphosis really is central to the mechanic" and players who want to do this content will simply have to accept that reality.
I'm not entirely convinced that's a great thing for any difficulty of raiding -- I've long suffered through heroic raids with less than ideal comps -- but if we're going to have it, at least confining it to the hardest content makes a kind of sense. Mythic raiding will be the status symbol raid in Warlords, and so design choices such as whether or not to include unique mechanics that very specific classes have counters for is more palatable than doing it in LFR or normal difficulty.
It also does make use of Mythic's other big change - as the only raid size in Warlords with a specific cap -- 20 players no more and nor less -- and makes that decision one that carries its implications forward. Designing heroic modes for both 10 and 25 meant that specific edge case abilities couldn't be counted upon - the only way to really challenge 10 and 25 was with increased damage and by making certain mechanics scale (1 orb on 10, 3 on 25 sort of changes) that felt weird and often more difficult for one raid size than another. With mythic having that fixed raid size, not only can we count on the raids having a much higher chance of having that specific class with the unique ability, we can fix how we challenge them. More unique Mythic only mechanics can be designed, free from the worry about how to make them scale up and down for a specific number of players.
It's definitely something I'll be paying close attention to in Warlords - how many times does a Mythic difficulty fight use something outlandish like this, and how do they choose to increase difficulty? These are the kinds of design choices that ultimately change the game.