I played & loved GW2. My biggest gripe with the game (the ones that made me stop) was this:
The difficulty difference between outdoor quests / events / whatever and 'explore mode' dungeons was ridiculous, and you were barely awarded with better gear for venturing into dungeons. Since my guild insisted on farming these for mostly cosmetic upgrades, and none of them were very good players, many of my evenings out with the guild were chain-wiping through silly hard content. Dungeons in GW2 break down like this:
'Story mode' dungeons: you get to experience the main story of a dungeon. Nice to do once, but kind of faceroll. Terrible loot, same quality as outdoor quest rewards.
'Explore mode' dungeons: the same dungeons, broken up into multiple wings, with new bosses and events. Painfully difficult with an inexperienced group (although probably faceroll too with a good group). These offer new items, but practically all of them are cosmetic upgrades only.
'Fractals' are short dungeons which finally do offer very tiny 'real' upgrades. These were boring, repetitive, and similar to D3 with its MP levels (you determine your own difficulty setting).
So in the end, the lack of fun I had while exploring dungeons kind of killed the game for me. Dungeon delving is the most important thing in an RPG for me, and it is an experience that WoW, D3 or even Baldur's Gate deliver better than GW2. Granted, GW2 offers a lot of dungeon content, but none of it feels well balanced in terms of difficulty vs. reward, so much so that, if I were to come back to the game, I would build a character purely for outdoor event farming (which is insanely rewarding for the time you invest).