The problem is usually not enough content. When there aren't new things to learn and master people default into "speed clear efficiency mode" and try to do everything as quickly and efficiently as possible. In my opinion people get stuck in that mentality and then continue it over to everything. Some people actually think like that and like to play like that, but most just get carried along under the mantras of "efficiency, speed, and optimal play". You can play however you want, but the upper limit is clear and easy to see and after a while most players won't want to feel handicapped any longer.
Beyond that mechanics have to be consistent and differing enough to make one character different than the next. That's all easier said than done.
The openness of builds is great for engaging players, but when they can't actually use that open system there isn't much of a point. There's a certain logical trap that most fall into that others are experts and they should do whatever the experts say in order to be good. I've been in all kinds of groups in all kinds of game and the authoritarian types usually don't do well. The best groups are open to working together. In GW1 at the very start that meant changing your own build to fit the needs of the overall group. Later people began refining their roles down to every step and players in the group were replaceable cogs. Quickly being a necro meant actually being an MM, being a Warrior meant being a tank, being an ele meant being a nuker, and so on.
In GW2 there is a quite open group and build system, but the openness in builds doesn't really matter. In the end it doesn't matter because all you can do is dump damage on whatever problem you are facing and avoid incoming damage with dodging and interchangeable mitigation skills. The mechanics overall are all the same and interchangeable now regardless of what profession or build you play, instead of the player. Throw in a tendency to need the zerg for most new content and your personal contribution is impossible to distinguish.