One is multiplayer, the other isn't. It's as simple as that.
In one game, you can show off your gear to your friends and hang in a raid, run Trials together, and even hit up a few strikes.
In the other game, there's no showing. None of that hanging out. It's just you and your friends in a party, [i]talking[/i] about how you're playing in your own little worlds. If you're a loner to begin with, that's okay too. But for the large percentage of Destiny's playerbase, I'm willing to bet you guys won't stay away from this game for very long...
Now don't get me wrong, I'm pumped for Fallout. And I agree with everyone who's saying it'll be GOTY. I just don't think it will "obliterate Destiny" as much as everyone thinks it will. And personally, I'll still find myself compelled to do raids, nightfalls, and Trials every week.