Destiny's plot revolves around humanity's quest to recover the artifacts of the Golden Age and find redemption in the post-apocalyptic wastelands that its brazen rush for advancement has created. Dague explained that Destiny is about making choices, with one of the most important being the class you decide to play. Whether it's the tanky Titan, the stealthy Hunter, or the magic-y Warlock, class selection will determine how you interact with the world, and groups with mixed classes will see greater success.
According to Dague, the dynamic world of Destiny and its reliance on inter-player reactions ensure that no two playthroughs are the same. Bungie is expecting the game to have a very long life cycle due to the idea that "the game is completely dynamic and will be different every time you experience it with other people."
Also mentioned was Destiny's focus on dynamic group events and the way in which players choose to deal with them.
If you're on your own, you may see a public event and decide that you want to jump in and participate in it, or you can bypass all of the actions over here and continue on your own path. You're going to see other players participating in their own portions of the game and you can decide to participate or just go your own way.