When I think back to what kept me playing GW1 so much, problem solving was a big part of it. (For the record, I'm not much of an MMO junky, so comparisons to WoW and most others are largely lost on me, sorry!). In GW1, there were all kinds of places where you could be super-powerful IF you could work out good counters to the enemies skill sets, or control their positioning. That made the game mostly about your ability to understand and adapt to challenges. As it turns out, any game with a decent concept of strategy does it the same way (and I don't just mean video games here). Why is MtG still so popular after being out for 20 years? In part, because of this very idea (I suppose collectibility helps some too). People have been playing (and enjoying) chess for hundred of years, despite simple rules because of the depth of "problem solving" required.
What "problems" in GW2 (and I don't mean bugs, design flaws, etc.) can't be solved by simply adding another person or two to the zerg? There are very few things I can name off the top of my head in the game where actually following a strategy is required much over button mashing. Sure, strategic play may work out to be more efficient in some ways, but there's not a very big incentive for it, let alone a requirement as far as I can see. Contrast that to something like fighting Mallyx in DoA, or Dhuum in the UW... big difference.