[Not sure how I feel about this: Bringing the Kotel to You at Life in Israel]
I saw the following in Christopher Hitchens' 2004 bashing of Michael Moore on Slate. It's an attack on Moore's style of documentary, but it's also a very good point regarding the way a rabbi sermonizes:
I know, thanks, before you tell me, that a documentary must have a "POV" or point of view and that it must also impose a narrative line. But if you leave out absolutely everything that might give your "narrative" a problem and throw in any old rubbish that might support it, and you don't even care that one bit of that rubbish flatly contradicts the next bit, and you give no chance to those who might differ, then you have betrayed your craft. If you flatter and fawn upon your potential audience, I might add, you are patronizing them and insulting them. By the same token, if I write an article and I quote somebody and for space reasons put in an ellipsis like this (…), I swear on my children that I am not leaving out anything that, if quoted in full, would alter the original meaning or its significance. Those who violate this pact with readers or viewers are to be despised.
Two points here:
1. Certainly, a derashah must have a point of view. However, a derashah which only presents that view and fails to point out and address opposition is simplistic and dishonest.
2. Sources must be cited honestly. Quoting half of a midrash or a commentary because that supports your contention, but omitting the rest without so much as pointing out the problem, is, indeed, dishonest.
A good derashah, in my point of view, acknowledges complexity and addresses it with nuance, and uses sources without abusing them. Writing a good derashah is hard work. No wonder my Thursdays were such nightmares when I was in the rabbinate...