This is something I've pondered as well, and an opinion I'm in agreement with. Verdant Brink in particular is honestly mind-blowing -- I still remember playing through it for the first time like it was yesterday.
You touched on how great the art design is, which it definitely is, but it was the intricacy of the map design that I found fascinating, particularly for an MMO. The verticality and layering made it feel like a hostile environment regardless of the enemies -- traversal was difficult, but rewarding. I also think it was really unique how they incorporated some Metroidvania-like design with the masteries, where you could later access areas that you may not have been able to reach previously. Verdant Brink really is one of the few examples I can think of where the environmental storytelling and gameplay are so cohesively informed by the broader narrative beats, that it feels like one cohesive package.
There have been times where I agree that it has felt like Anet were at the top of the pile when it came to innovating in RPGs/MMOs (the first 3 Guild Wars games come to mind for me), and I think HoT was such a time as well.
While I do think Anet still put put out a good product, another factor to take into account is how many senior staff have left/were laid off over the years. When individuals with years of institutional knowledge leave a studio, often times they simply can't be replaced.