Chasing a target is invigorating, especially if there’s a reward for it.
If you answer 10 posts you might get a badge, if you answer 100 you get to create blog posts and help moderate the community etc…
Individual targets like these encourage some very good behaviors (helping lots of people) and some very bad behaviors (cheating the system and publishing lots of low-quality posts).
A more interesting approach is to set group goals. You can set goals for either the entire community (i.e. if the community achieves [x], we will do [y] for the community] or for sub-groups within the community (i.e. form/join a team, create a great content piece, and your whole group is rewarded).
Which approach do you think best unites people, encourages the most contributions, and is the most fun for members?