[Thenamir]

I find the absence of hope to be central to several characters.

Theoden is a good example. Until Gandalf came along he was weak and ineffective because Wormtongue had sowed the seeds of hopelessness and despair for so long. Once Gandalf came along (wearing Narya, the ring of fire, given to him by Cirdan that he might rekindle hope in the hearts of Middle Earth) Theoden regains hope and is transformed into the warrior king he was meant to be.

The hopelessness of Denethor is crushing in its depth, and ultimately fatal. Sauron himself, via the palantir, was able to create fear, uncertainty and doubt (the FUD factor) in Denethor to such a degree that even Gandalf could not save him.

This is a good area for further study. (Thenamir fires up his online copy of LOTR and starts some searches...)