I find the number of incomplete profiles on the site to be a weakness. When considering an answer from, or a reply to another user, the profile can provide important information that enables a more user-centric approach to the task.
Backgrounds, experience, and locations all add value to the experience of using the site to communicate with others. So it's a real bummer to regularly click through to a profile page only to find the cupboard bare.
I put forward for consideration the idea of incentivising profile completion by awarding a small number of points for doing so.