Seven Pillars of Design
From Destinypedia, the Destiny wiki
The Seven Pillars of Design are the guidelines for Destiny's development, as told by Bungie. Somewhat of a mission statement, the pillars are intended to keep the team focused on the important concepts and ideas that make Destiny. 
Pillar 1: A World Players Want to Be In
Jason Jones: "This pillar really influenced us early in development. Is this world cool? Do I want to stay here? Do I want to learn more about it?"
Joseph Staten: "One lesson that's critical is that the most important stories we tell aren't going to be told by us. They're going to be told by players – their personal legends built from shared adventures."
Pillar 2: A Bunch of Fun Things to Do
(after describing a three-man dungeon run in detail):
Joseph Staten: "And just like that, The Dust Palace becomes part of my story. The breadth and depth of Destiny's world encourages me to find my own adventures. It's a place where I can leave my mark."
Pillar 3: Rewards Players Care About
Jones explained how the game will have "a lot of great things to earn, find, and make," reiterating that "everything you do in Destiny earns rewards." Besides unique weapons, every piece of your kit will be your own, from your helmet to your cape to your armor pieces to your face. Their goal, he said, is to keep players coming back "day after day, week after week, month after month, [and] year after year."
Pillar 4: A New Experience Every Night
Jason Jones: "Imagine you could spend an hour and accomplish something, but you get distracted from doing the thing you meant to do when you logged on. Every time you sit down to play Destiny you have a different experience than last time."
Jason Jones: "Everything that's fun to do is more fun to do with your friends.'"
Pillar 6: Enjoyable By All Skill Levels
Jason Jones: "All core activities can be enjoyed by a novice player, that's not hard. What's hard is keeping it interesting for your advanced players."
Pillar 7: Enjoyable by the Impatient and Distracted
Jason Jones: "Players don't want to work hard, they don't want to read, and they don't' want to go to the Internet to figure out our bullshit."
- The amount of pillars is a reference to Bungie's favorite number, 7.