Are you planning:
- follow tags, categories, people?
- uploading photos?
- writing anonymus questions but with registration
- posting questions to facebook after posting it in the system?
- see facebook friends that are using the site and follweing them?
- Internal messaging system between registered users?
- a list of popular users?
- showing related questions after first step of posting a question? (so users will not repost questions that are in the system_)
- support in sub categories