Essentially, there are 3 difficulty tracks. We can call them softcore, midcore, and hardcore. Here's how each MMO handles the difficulty tracks:
|World of Warcraft||LFR, Normal||Heroic||Mythic|
|The Old Republic||Story||Hard||Nightmare|
|FFXIV: ARR||Crystal Tower (24), Hard Primals||Binding Coil (8), Extreme Primals||Savage Binding Coil|
|FFXIV: Heavensward||Void Ark (24), Alexander (8)||Extreme Primals||Savage Alexander|
The softcore layer is also the layer that allows for random transient groups.
Each of these games have issues with the layers, though different issues.
WoW's major issue is that LFR is a complete joke. As well, it's essentially the same content repeated multiple times. Finally, because Mythic is a fixed size, and Heroic is not, the transition between Heroic and Mythic can be hard to navigate.
TOR's major issue revolves around Nightmare modes. In theory they were supposed to give better gear than Hard modes. However, there's been a "bug" which means they drop the same gear. Bioware has also announced that the fix is to give Nightmare a "chance" to drop better gear. This is primarily to people from getting the best gear too fast. But there is one highlighted Hard mode each week which drops guaranteed best gear. So TOR is in the awkward position of having a Hard mode instance guarantee better gear than the Nightmare version, which has the hardcore players unhappy.
Additionally, it's been over a year since a new TOR operation was released, so most players in the midcore and hardcore streams have done the current content a lot.
For FFXIV, in ARR, Binding Coil was out of reach for the softcore tier, which is composed of the largest number of players. In Heavensward, SE tried to fix that, moving the 8-man raid into the softcore tier. Unfortunately, that hollowed out the midcore content. The midcore are left with throwing themselves futilely at Savage, or just settling for the easier softcore content.
As well, the way loot was handled damaged this structure further. The current Extreme Primals had their rewards obsoleted really quickly.
Anyways, that's a look at the current state of endgame difficulty in PvE MMOs. I confess that I liked the FFXIV:ARR system the best. It struck a good balance between repeating content and making content exclusive. I've never actually done Binding Coil, but I think there was enough other content that it never felt necessary.