Denethor's madness.

[Beanamir of Gondor]

This might sound naive, but in the chapter The Siege of Gondor in ROTK, which I read waaaaaaay before I ever saw the movie, I really saw Book!Denethor as a thoroughly unpleasant character. As soon as you meet him, you feel his distaste for Gandalf, his coldness towards Faramir ("Yes, I do wish that! For Boromir was loyal to me, and no wizard's pupil!"), and his utter obsession with Gondor. I only really started liking Book!Denethor AFTER he went nuts and silently sat at Faramir's bedside, white-faced.
So in one respect, I think that PJ messed up: Book!Denethor was unpleasant, all right, but essentially he really did love Faramir. That never, ever made it into Movie!Denethor's character, even if Gandalf said it out loud. That little putzy "Faramir?" at the end, right before he jumped off the Wall in flames, was nothing like what Book!Denethor said or did. I feel a little bit the same way about Movie!Denethor as I do about Movie!SiriusBlack.

As far as I'm concerned about Gandalf beating up the Steward, that was comic relief. Gimli wasn't there, so it was up to Gandalf.