Ability to specify custom flag reason/explanation
Related: Feature Suggestion: More Flags(!)
On other Q&A sites, I sometimes "meta-flag" posts; that is, I will flag a post that might not have anything obviously wrong with it in order to point out questionable activity by the post's author or to indicate that the post might be part of a pattern.
In order to facilitate this, it is necessary for me to be able to specify a custom reason (via free-form text input) why I am flagging a post.
Askbot does not currently have this feature, however.