This is an interesting article by Mike Gilbart-Smith at 9Marks. Some quotes:
Mark Dever rightly describes Expositional Preaching as "preaching that takes for the point of a sermon the point of a particular passage of Scripture." However, I have heard many sermons that intend to be expositional, yet fall somewhat short. Below are seven pitfalls that one might try to avoid. Each of these pitfalls either doesn’t correctly make the message of the passage the message of the sermon, or doesn’t make it a message to that congregation at all.There's five more points. Go check them out yourself.
1) The point of the passage is misunderstood: the ‘Unfounded Sermon’.
This is where the preacher says things that may or may not be true, but that in no sense came from the passage, when understood correctly. This can happen either by carelessness with the content of the text (e.g. the sermon on "production, prompting and inspiration" from the NIV of 1 Thessalonians 1:3, though each word has no parallel in the Greek) or carelessness with the context (e.g. the sermon on David and Goliath, that asks ‘who is your Goliath, and what are the five smooth stones that you need to be prepared to use against him?’).
If a preacher is not deeply mining the truth of God’s Word to determine the message of his sermons, they are likely being driven by his own preferences. For "When someone regularly preaches in a way that is not expositional, the sermons tend to be only on the topics that interest the preacher" (Nine Marks, 41). Thus the congregation doesn’t receive all that God intended. The lesson? Preachers must give themselves to thoroughly understanding the text before setting out to write their sermons. A cursory reading is not enough. Preachers must allow God to determine the sheep’s diet so as to prevent an insufficient feeding.
2) The point of the passage is ignored: the ‘Springboard Sermon’.
Closely related is the sermon where the preacher has understood the center of the text, pays lip service to it, and then becomes intrigued by something that is a secondary or tertiary point, fixing his attention on that for the remainder of the sermon. What he says does come from the text, but is not the main point of the text (e.g. the sermon on John 3 that focuses primarily on the lawfulness of Christians drinking alcohol).