Incredible, unrelenting fatigue.
Constant sense of hopelessness and helplessness.
Loss of interest in things that normally would have kept me interested.
Inability to concentrate for more than very short periods of time (notice this mostly in reading)
Sleep disturbance (sometimes insomnia; sometimes excessive sleep)
Deep despair; often resulting in suicidal thinking

(These are pretty much the defined characteristics of major depression. They seem to fit me completely when I am depressed.)