Two features that stand out to me the most are email notifications, and advanced search features.
You don't get an email when someone sends you a message, or when someone comments on your post in the Forum, or really anything else related to your account here. If users were emailed for activity, they might log in more often.
I think advanced search features is obvious. You should be able to narrow down users by more criteria than just age, sect, and location.
What else can you all think of?