Bokeh characteristics are primarily determined by the circle of confusion. In out-of-focus areas, each point of light is instead a disc. In some lenses, that disc is uniformly illuminated, for others it is brighter near the edge, and for others it is brighter near the center. Some lenses show one kind of disc for out-of-focus points closer to the camera, and a different kind for points far from the camera.
The characteristics of a lens that cause it to produce pleasing bokeh are not fully understood. Generally lenses with more blades in the diaphragm tend to produce more pleasing bokeh, however this isn't always the case. Zoom lenses often have poor bokeh but again, this isn't always the case. Mirror lenses produce a "doughnut" bokeh that is generally considered as umpleaseant. Pleasing bokeh is often very important for wide aperture lenses because they have a shallow depth of field at maximun aperteure. Bokeh is also important a portrait lens (medium telephoto) because the photographer very often selects a shallow depth of field to make to have a a out of focus background and make the subject stand out.