See FAQ on deleting on meta.SO: http://meta.stackoverflow.com/questions/5221/how-does-deleting-work-what-can-cause-a-post-to-be-deleted-and-what-does-that
In particular, you can read there that:
- A question, that has an upvoted answer or has multiple answers cannot be deleted.
- A question can be only deleted by the poster, a moderator, by several users casting vote to delete or, in some occasion, by the system. (Probably the most frequent kind of questions deleted by SE software is an unanswered question, which has negative score, is deleted after 30 days. Another thing you can see rather often is automatic deletion of migration stubs after 30 days.)