Well this is all just my opinion and probably mostly driven by nostalgia, but gw1 to me feels like the story is more about you/your character, and your friends(as opposed to the world). Sure gw2 has instanced content, but it feels more like a traditional MMO than gw1 does. There's nothing particularly wrong with that, but literally all of the mission and quest content in og guild wars is instanced, so it feels more like a single player RPG that you can play with friends and less like an mmo unless you are in a town where you can party up and trade with other players. It really captured a feeling of being out in the wild by yourself and didn't have the same feeling that I get playing every other MMO where you just feel like your going down a beaten path, even if you technically were doing something that literally everyone else was doing. Some games like PoE do have a similar system but they do not do their storytelling or combat in quite the same way.

There's a bit more to it than that though, the art style, the combat mechanics, and the tone of the story all changed quite dramatically between the games. Gw1 has a less exaggerated, more grimy and down to earth artstyle compared to the more "paintbrush" artstyle of gw2 and gw2 also feels like the overall tone is more lighthearted and gw1 wasn't afraid to just strait up off npcs that you grew to know well in order to advance the story and up the stakes.

Playing gw2 for the first time as a long time gw1 player was a strange experience, because it felt like Anet had abandoned many of the things that made guild wars unique and different compared to standard mmos, and it honestly felt like they took the stock mmo formula, applied a vaguely guild wars artstyle to it and removed healer classes.

To be clear I'm not saying gw2 is a bad game, just that it's very different from og guild wars. I would not be surprised if gw3 is significantly different than gw2 and 1.