Speaking at a recent Best Buy Google hangout, Bungie's community manager David "Deej" Dague stated that players are not likely to finish Destiny because of the sheer size of the world and the amount of content in the game.
Here's the full quote:
"We saw people invest dozens of hours into the beta alone, and that was one destination. These destinations are stitched together with our new tech so that there are so many different locations to visit. Expansive outdoor locations where you can just enjoy your personal vehicle; there are indoor dungeon-type spaces where you can really pick through the bones of our shattered civilization."
"If you’re a speed-runner, and all you want to do is blaze through and complete one story mission after one story mission, you’re really just scratching the surface, because the Strikes add such an extra level of complexity."
"And as we said before, [there are] endless amount of hours of fun in the Crucible. Until you’ve kicked every single ass that plays this game, you haven’t finished. And once you have, someone else just bought their first copy. So, fresh meat, Christmas noobs."