Bipolarity. Unipolar depression is characterized by psychomotor retardation, loss of interest in usually enjoyed activities, amotivational periods. The flip side is periods of extremely active, often extremely productive activate, as seen by such people as ted turner. If one has bipolar depression and it qis treated for what presents as unipolar depression, the manic phase can rear it's scary head.
Mixed Depression. Could be a combination of anxiety and depression, or another state where two forms of depression occur at the same time, such as major depression and dysthymic disorder. There could be aspects of bi polar disorder as well.