Well, I've been playing Elementalist & Engineer since release. That challenge has to a large extent disappeared, even in the most challenging builds. If you dislike open world events, the only challenge left is Fractals and the occasional Lupicus-fight.
Then play something else for a bit and come back. I don't mean that in a trolling-fashion either. I've played WoW since retail release. I've taken many, many breaks, because the content just gets old. And then they come out with an xpac, i do the xpac with like 5 level 90 toons, and i'm done. I stop my sub and go play something else, like GW2 or Borderlands 2 or whatever your poison is.
Some people can play a game for YEARS and never grow tired of it. Some people get bored, move on, and either come back or never come back. Thats just how it is. No one is telling you to give away all your stuff, delete your toons, and uninstall the game from your PC.
GW2, like all things, must be taken in moderation. Otherwise, you burn out and/or get bored with it. I play like a demon at the moment, because im just fresh out of the honeymoon phase with the game. So, i'll be to that point soon enough and go on to the next big thing. On that token, i don't get mad ANet doesn't provide enough challenging content and etc to keep enthralled for the rest of my life.
Yes, the cycle can suck, but the great thing about games, is you're not married to them, so you're free to flirt and play any other game out there! lol