1- No forms of difficulty scaling.
2- The way your professions were designed... https://massivelyop.com/2019/03/28/massively-overthinking-thoughts-on-the-holy-trinity-in-mmos/
“Brianna Royce (@nbrianna, blog): Fun fact: I still remember when “holy trinity” meant tank, healer, and mezzer – the DPS players were a given, the warm bodies that filled out the rest of the group, and not part of the trinity back in the early pre-WoW days of MMO group content. The fact that this shifted over time really says all you need to know about how MMO class and combat design have changed, and not necessarily for the better.
Don’t mistake me; I no longer believe we need or must respect a trinity of either type. But what I truly resent is the loss of class variation and combat flow that naturally accompanied the demise of the classic trinity, specifically the fact that crowd control, buffing, and debuffing classes have all but disappeared in the modern rush to make nearly everyone a damage-dealer, even the healers and tanks.
As an example, I can still think of none better than City of Heroes, which offered all of the old trinity and new trinity class types (and then some) but made none of them actually mandatory to clear content. Yes, tanks and healers and CCers and buffers and debuffers and damage dealers all existed, but it was completely possible to get through the game with no healers, or all healers. With a scrapper tanking ahead of a fleet of corruptors. With a stalker and four controllers. With three bubblers and three tankers. Whatever. I don’t want to see strict trinity MMOs, but I’m even grumpier about the “everyone deeps” MMOs even more, especially when the end result is cluster**** combat where nobody ever has control over the fight. It didn’t have to be that way, but modernish devs keep reinventing the wheel, convinced they can do better. Maybe someday, they will, but so far, nah.”
Note- that "cluster" comment was a link to the GW2 section on MOP.
3- Combining number 1 + 2 ultimately created a toxic environment for instanced content that most people don't want to be part of.