What makes WoW raid encounters difficult is not that pushing healing buttons is harder than pushing a dodge button at the right time, but that GW2 is roughly at the encounter design stage of early/mid vanilla WoW, which were largely relatively straightforward execution/gear checks (where much of the complexity came from herding 40 cats). The major advance in encounter design in WoW since then -- aside from more multi-phase bosses -- has been the addition of multiple concurrent encounter mechanics. Dealing with multiple mechanics concurrently means multitasking, which humans are generally not very good at natively, and so that has to be learned and mastered for every individual encounter, until dealing with multiple encounter mechanics simultaneously or in quick succession becomes second nature.
The classic example was the combination of Val'kyr Shadowguards and Defile on the Lich King (or, on heroic mode, Shadow Trap + Necrotic Plague management, too). In short, more recent WoW raid encounters rely much more on multitasking/coordinating through (often multiply incompatible). E.g., on the Lich King, Val'kyr Shadowguards required you to stack up near the center of the platform, while Defile required you to spread out away from it, and you didn't know in which order it was going to happen for the second one. Lei Shen is also full of such mechanics (as has been pretty much any fight since T11). This is in contrast with vanilla WoW raids (or even current WoW dungeons, which for the most part, are a far cry from current WoW raids), which were/are largely about not standing in stuff, avoiding one-shot mechanics, or maximizing DPS/healing (a lot of vanilla encounters were also made difficult by simply having insufficient mechanics, such as aggro-trading on untauntable bosses without a threat meter, or the limited multi-mob threat of tanks, but that's a different story).
There's little reason to believe that more complex encounter mechanics couldn't be adapted to GW2; whether ArenaNet wants to (or is willing to invest the effort to design, debug, and test these) is of course another story.