Here's an example of a lens with good bokeh, the Nikkor 105 micro, 2.8. Notice how the branches in the background have a very soft, rounded, and aesthetically pleasing blurred appearance.
Same lens, great example of good bokeh (it's the lens, not me!)
I don't have any lenses with bad bokeh, but I'm sure you can find an example on Ken Rockwell's site. He compares several Nikon lenses to show the difference:
Nikon Lens Bokeh Comparison
Hope this helps.