In the ViDoc there was a shot of what looked like a squad menu with options on the left. One of them was titled Bounty and the description said Collect your weekly bonus with a countdown (5d 16hr...) I'm assuming this will be Destiny's version of weekly challenges. Bounty might reward us with things other than xp (weapons, armor, etc.) which would be a nice incentive for players to travel back to areas they've already been to. Also, faction-specific quests and side quests might have you exploring different areas of the same planet to keep things fresh.