For example early in the Mist of pandaria beta, your ability to complete a zone would break because when a bug broke any one of a number of quests on the main path through the zone, it was quite likely you the next quests wouldn't be unlocked.
Guild wars 2 seems to have three tiers of quest.
There's "heart" quests, where a particular area will have a set of actions that contribute to the heart completion. Different actions might contribute different amounts to the completion, but if you only find one action, you can still complete it.
The next tier is the "dynamic events", as you're wandering around events will happen, if the players complete the event a new event may open up. In the sylvari (elf/fae-ish) zone, an NPC will periodically decide it's time to relight the lamps, so she has you go kill fireflies to get luminescence. If enough is collected she'll walk over to the lights and "fill" them. A bit after the lights are "filled", mosquitos will be "attracted to the lights" and you'll need to defeat them.
The final type of quest is the "personal story". This is most like the WoW quest in that you get sent to talk to a person with a funny symbol over their head need to do what they ask for the story to continue.
The difference from WoW, though is the personal story is seeded from choices you make during character creation, it does appear to have some branch points. Each step of the personal story is instanced so, it really is all about your character, if your friends tag along they don't get to make decisions for your personal story.