It’s a pretty fundamental philosophical/design difference.
WildStar has been envisioned from the beginning as being a game that was trying to feature the kind of difficulty that existed in MMOs (even the leveling game) prior to WoW’s expansions. The concept was to create a game that was, from the very beginning of it, tailored to meet the needs and desires of the most hardcore, high-skill-cap players in the MMO community — a group which often feels like its needs are overlooked in MMOs these days. So, WildStar from soup to nuts is catering to the most intense, hardcore, high skill gamer — that’s it’s own niche. I think it will be successful in that niche, but there are many other players who play MMOs these days who are moderately skilled at best and are not hardcore — they will be punished by WildStar’s design, and will not find it rewarding —> and, that’s okay because they are not WildStar’s target audience. The most hardcore, skill-cap oriented players are what they are going for.
GW2 has a completely different philosophy behind it — the idea here was to appeal primarily to the casual gamer by eschewing (for the most part) endgame progression gear treadmills, locking substantial content up in parts of the game reserved for high skill-cap or high time-cap players, and so on. It’s a design that, from the get-go, was certainly destined to alienate the hardcore, high skill cap contingent of the MMO community, precisely because the entire design here is not something that revolves around their needs, but rather the more average, moderate skill, casual player who wants to play for a bit, have some fun, have easy seamless grouping, good QoL features and so on, and then log off, without feeling pressured to do anything in particular to keep up with everyone and mot be left behind the curve and become a social laughingstock in a video game.
So, as far as I can tell, you can’t really get two games that are more philosophically dissimilar than GW2 and WildStar. Almost 180 degrees different in philosophy and intended audience.