Mike Johnston, in his short publication "Lens Bokeh Ratings," states the following generalizations about bokeh (out-of-focus blur):

I’ve found as a general rule that bokeh gets progressively more problematic:
• the larger the aperture
• the closer the focus
• the more distant the background
• the more contrasty the background

I think that when we discuss which lenses produce nice OOF blur (bokeh) compared to those that don't, we too often compare lenses under the worst of circumstances. For example, here are two shots of my sons with the 50/1.4 under nearly the worst of circumstances (Click on image for full size):





From these, one might conclude that the 50/1.4 harshly renders OOF blur (bokeh). This is far from true. Compare the OOF blur from the 50/1.4 at f/2.8-f/4 with any 50mm lens irrespective of price range, and it will compete well. Conversely, take the last generation pre-aspherical 35mm Summicron-M and shoot it wide open with a close subject and a distant, contrasty background. Even the so-called "King of Bokeh" will render a harsh OOF background blur under such circumstances.

Now see how the 50/1.4 renders an extremely challenging background at f/2.8:



Significant improvement. Now take a few moments to consider William Castleman's bokeh comparison of the 50/1.2L and 50/1.4.

An interesting consequence of this effect of stopping down on the quality of bokeh - independent of the quantity of background blur - can be seen in this bokeh comparison between the 35L on the 30D and 50/1.4 on the 5D.