My biggest problem with storytelling in MMOs has always been that you can't affect the world. You're going along, saving the world or whatever, and you finish a mission. Then when you're done, you look back and see another group of people doing [i]exactly the same mission.[/i] It kills the suspension of disbelief. Your story advances, but the world stays the same. I feel like the point of "massively multiplayer" is kind of lost if you're just doing the exact same thing you would do in any other game, only with a large group.
Obviously, it poses a massive technical challenge to have any player affect the world in a meaningful way. In fact, it seems borderline impossible. But with Bungie talking all about "Becoming legend" and Destiny being a "living world" it really has me excited, that they might actually try what no one has been able to pull off. And if anyone is going to try the impossible it's Bungie.
In all fairness, I think CCP has done a pretty good job with EVE online, where large groups of players can actually take over areas, or influence battles somewhat.. wouldn't you say?