I'm really excited to see Anet release the Story Journal, a system that changes how people will see GW2 forever.
- Story missions aren't temporary anymore
- The missions are unlocked (by simply logging in during the release of the update) in your story tab
- each mission can be replayed as often as you want
- gone are temporary meta-achievements, instead each mission has permanent (+ more challenging, catered to the more hardcore crowd) achievements
- including corresponding rewards
It's important to know that Anet sees the living story as GW2's endgame (quote from Colin in the Dev-Talk linked below). That's why the story (which is accountbound now) is only playable at lv.80. You can unlock it earlier though and save it for later. It makes sense since the achievements tied to the missions are considered challenging.
Should you miss to login during the 2 week (some may be longer than 2 weeks) period, you can get the missed chapters in the gemstore for 200 gems or about 20 gold (which equals 10-20 dungeon runs). Should you log in even for a second, it's unlocked and completely free.
Furthermore we learned that most content in Season 2 will be permanent. Open world can still change in order to build up to upcoming chapters (see: Southsun Cove) but from what I've learned the core content will stay.
After season 2 the devs will talk about / thinking about how to bring back season 1 in story journal format. It's can't be done at the moment since season 1 wasn't developed with the story journal in mind.
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and a much better and more complete summary by Dulfy: