Completing All Your Bounties so You Have Room for More
Bounties are most useful under level 40, as they typically grant 2,500 to 3,000 XP but completing them can still yield valuable resources, help you find new engrams, and improve your Vanguard and faction reputation even when you hit the level cap.
Some can be tough, with very specific conditions attached, but many are pretty straightforward ways of advancing through the game. The Taken King makes them even easier, as you no longer have to return to the Tower to turn them in once completed.