Ok. So my brother and I were talking about this and wondered how long will destiny be truly fun? We already know reaching the level cap of 20 will not be too terribly difficult and replaying old areas once your a higher level is not nearly as fun. Borderlands solution was enemies level up alongside you but that isn't really possible in online game such as this. So when the story is done, you've reached the level cap, and crucible has lost it's charm, what then? DLC's maybe?
It's called raids strikes and crucible and when I reached the lvl cap in the beta all of my skill continued to level up so I fell like I was making progress with my character anyways and considering that there will most likely be 3 subclasses for each class(one at start one at lvl 15 and I am guessing that there will be another one at lvl 20) your character will still progress after lvl 20 and remember weapons and armor can be upgraded have certain perks and reduce cool down time for for your Malay/super/grenade. So if you want to have your grenade recharge really fast you'll be on the hunt armor that has grenade cool down (discipline) and gives you bonus to the preferred weapons you use [spoiler]I believe that you will be occupied for a really long time [/spoiler]