Yeah, I was kind of having the same thought earlier, having had similar issues with Borderlands 2. It's a great game as long as everyone's the same level. If you're a few levels apart though, you can forget any good gameplay - until you catch up, all you're doing is power-leveling the lower character.

On the other hand, it's possible to build a game that doesn't have those issues. What if in Destiny they aren't character levels, just different gear? As long as the power afforded by any one piece of gear isn't too far out of line, you could feasibly have progression accomplished purely through finding the sort of gear you want (and possibly any abilities would also come not from leveling up, but from found/equipped gear). Think Halo gameplay with Mass Effect 3's item/weapon system (not the frustrating randomized gear packs, just the concept of having more powerful weapons be balanced out by limited ammo or fire rate).

... it shouldn't be a problem. During the presentation to the media they talked about Jason playing a game with Joe and Jason was much higher level than Joe. Now it is still of course to early to know for sure, but it seems they pointed that out purposefully as if to suggest it wouldn't be a big deal.