Bungie thinks it's all about getting players invested with broader experiences and a living community.
In a recent podcast, Investment Lead Tyson Green said that Destiny is aiming to stick around: "Investment is not about experience points, [and] it's not about item drops; it's about having an enduring attachment to a game and to a community so that that community can exist and thrive. If you unlock new abilities and you give people different weapons that are balanced but different, then those are valid rewards that don't change the game or ruin the game but [...] broaden the game, [...] give people more interesting opportunities, and give people more varied experiences."
These varied experiences, according to Green, include rewarding players for actions outside of the norm and encouraging social gameplay. Bungie hopes that you'll play the game not out of obligation but because you are invested in Destiny and its community.