Walter L. Liefeld suggests that an expository message:

1. deals with one basic passage of Scripture.  One passage of Scripture (or a very few, I would suggest) is accorded central place in the sermon.  The sermon is not a ‘string of pearls’, consisting of a diverse texts.
2. has hermeutical integrity.  It presents the gist of the chosen passage, consistent with its original meaning and emphases.  The sermon must be true to the genre, original purpose, flow of narrative or argument, intended meaning and application of the text.
3. has cohesion.  It must not be an array of separate and unconnected thoughts or ideas.  The individual parts must relate faithfully to the whole.
4. has movement and direction.  The sermon must reflect the logical or chronological flow of the text.
5. has application.  And that application must be faithful to the purpose and meaning of the text itself.  Without application, it is not a sermon, but rather a lecture.

(Abridged and slightly adapted)