We all know that a user story should not aim to capture all the detail about a requirement or business need. It is more of a promise that a conversation will take place, the options explored and some work will be done that will deliver that business value.
One of the key things to make sure of, when writing user stories, is to have a clear view as to when a piece of work is completed. The normal way of doing this is to document a set of acceptance criteria against a user story. What we don't want though is to write a 20 page requirements document as acceptance criteria.