Exactly, at the risk of coming off as extremely arrogant (and not just elitist) I'll expand on this. As I said earlier, people take every role into account when they talk about raiding difficulty. Let's say there's 3 basic mechanics in an encounter. A taunt rotation, a move out of voidzones mechanic and lastly an anti-overheal mechanic which does AoE dmg equal to the amount you overheal divided over the entire raid. What people tend to do is say "Yea there's this fight where you have to keep track of several stuff: you need a taunt rotation, avoid void zones and you can't overheal!" and they forget that at most you'll need to watch 2things (voidzones + overhealing/taunting).
You expanded on this and went with phases where the same thing happens. Now imagine a fight with 3phases (+2 transition phases) where every phase has similar mechanics to the one described above, people now say "Yea you have to look out for like 15things!".
A while ago I illustrated this by using HC LK as an example.
On a similar note, I don't understand why people need VOIP for PvE content. I used to log into vent because it was required and then I'd mute it and have friends PM me if someone was talking to me. I even raided without DBM since the in-game timers became really good.
On vent the only thing that happens is someone says "go here, go there, watch this, stop attacking, start attacking, target this guy" which is information that's available to everyone.
OHK-mechanics are sometimes needed to make fights significantly more "complex" imagine a fight where there's a window to DPS a boss but in return you take 10% of the damage you do as reflects, if you use cooldowns here you can totally negate that mechanic so to avoid this you put in a OHK before this mechanic so that the CD's are no longer available.
An issue pops up when you rely solely on OHK mechanics since they don't actually increase difficulty/complexity on their own, add to this that you'll always feel (whether or not it's justified doesn't matter) like RNG got the better of you while when you use them as I said before it's easy to identify what you did wrong on a personal/skill level.
As you probably noticed I mention OHK -and burnmechanics in one breath. The reason for this is that both are meaningless on their own, think of Patchwerk as an opposite example to the one above.
I personally quite like burnmechanics since they make people think on how to execute a fight. I remember having issues with hitting the enrage timer every single time on a fight and we were stuck there for a week or two after which we dropped 2healers for DPS and we did it like it was nothing, the only thing that held us back was our inability/dislike to innovate.
To link this to GW2:
The problem with difficulty/dungeons/raids/... isn't that a-net has less mechanics they can use, it's their design philosophy: "everyone can do our content". As we all know this one-size fits all sort of approach never satisfies anyone, the OSFA overalls don't fit anyone and everyone would like some thing different but hey that's not the idea of overalls.
GW2 had this idea of in-out fighting which heavily uses OHK mechanics and "burn times", you deal dps (in) until the OHK comes (out) which can be very dynamic, fluid and fast. The problem is that there's no real burn time where there's a consequence if you don't meet a threshold. What happens is that people can ignore either the in or out part completely and you can take that quite literally since the difference usually comes down to ranging a fight vs meleeing it.
The problem is that you can very easily take advantage of the AI in any game. Smart use of game-mechanics hardly ever is using special item X, it's usually stand here, stack/spread, ... If you're familiar with tank-wars there's a trick used by all top-scorers that takes advantage of the AI always wanting to go the shortest route, what you do is you make a very long labyrinth with an opening to your base and a short red-carpet like hall to a closed off entrance of your base. Once the labyrinth is full you delete the wall of the red-carpet hall and block of the entrance at the labyrinth and all the tanks will turn around increasing your time to deal damage (you can take this further by using multiple labyrinths and cycling through them all).
An example would be using skills that the AI would want to dodge to "interrupt" the boss' dangerous attacks (or vice versa).
For weak targets you'd have to hard-code specific profession targeting in which wouldn't be fun. Compare a mesmer to an ele for tankability. The mesmer has 2dodges, 2second evade on a 12s CD another immunity with distortion, blink, shadow walk while the ele "only" has armor of earth, teleport, mistform, arcane shield and rock barrier. So now I should always go for the ele which forces all eles to bring those skills.
A similar problem arose in GW which used a similar system based on pick-up items. If you were carying something of importance you no longer could use your (weapon) skills and every NPC would focus you, this turned into people stacking certain armor on one toon while the others raped the NPC's.
Your last Idea could lead to people intentionally dieing on certain parts of a fight. Imagine a boss with a bouncing attack which bounces to anyone within a distance of 10 kablamos. When you have 10people in that room it is impossible to stand in such a way that less than 4people are hit per attack (so it's 1person = target + 3 bounces) now if we let one person die (maybe a healer who we don't really need in this stage since less bounces = easier to heal) it is possible to only have 2people get hit (target + 1) bounce if you stand in a certain position. And after this phase you simply rez the sacrifice and continue.