Thanks for this thread. Ecclessiastes is actually one of my favorite books, but I've never preached on it...and I preach from the Old Testament a lot.
I guess we haven't heard it preached much because we're not quite sure of the message. It sometimes seems life-affirming, and sometimes seems deeply pessimistic. It's certainly wonderful to read just for contemplation. I like particularly - maybe because of my age - chapter 12. It's a contemplation of the phase in our lives where we're looking back to when we were young, and ends with a message that without God, life is meaningless.
Ecclessiastes 12:8, 13-14
8 Vanity of vanities, saith Koheleth; all is vanity.
13 The end of the matter, all having been heard: fear God, and keep His commandments; for this is the whole man.
14 For God shall bring every work into the judgment concerning every hidden thing, whether it be good or whether it be evil.