The quest for good bokeh and bad bokeh...haha
This might be an expensive quest.
Good bokeh is "pleasing" to the eye and bad bokeh is not.
Rule No. 1: good bokeh should be round.
i.e. points of light that are out of focus should be round and not distorted, oval, hexagonal etc. Typically this requires a great lens plus more aperture blades (= more $)
As drakon09 wrote: the aperture should be as close to a circle as possible
Rule No. 2: out of focus objects should look natural
i.e. objects that are out of focus should look natural with no additional artefacts/lines/distortion, this gives a natural 3D effect to the picture
Note 1: some lenses are very sharp but bokeh is bad. such lenses should only be used for situation where you want total sharp focus but not differential focus.
Note 2: some filters causes internal reflections resulting in bad bokeh
e.g. a single object or line may appear as two OOF (out-of-focus) objects
Multi-coated filters and lenses reduce this problem (= more $)
Note 3: good bokeh does not necessarily mean low DOF
i.e. you may still want the background or foreground to be recognisable to provide a context for the picture although you might want to draw attention to the focal point where the image is sharp
Note 4: typically good bokeh comes from big aperture lenses (= more $) although you might use slightly smaller aperture depending on what DOF you need,
e.g. 50/1.2, 85/1.4, 135/2, 300/2.8 etc
Note 5: Since you often need to stop down slightly to get more DOF, a great 2.8 zoom (e.g. trinity) can often give you a sufficiently good bokeh. (trinity = more $)
Note 6: The best bokeh should give you a 3D effect as in points slightly of of the DOF are slightly OOF and points further from the DOF is more OOF. And it should look natural.
Note 7: Photoshop can help in some situations but cannot completely replace good bokeh lens.
Note 8: Good bokeh should not be hard-edged or contrasty.
ie. a blurred point should not appear as a circle of light with sharp edge. The edges should be diffused.
Note 9: Analysing the MTF curves of a lens can give you the theoretical bokeh performance of a lens.
Hope we are less blur about this blur stuff.