Roundness of the aperture is relevant, but probably not as important as the degree of correction of spherical aberration. An overcorrected lens will often be very sharp, but can have "bad bokeh." Lenses with lots of aperture blades also happen to be older lenses, or in some cases newer lenses aimed at a market that is interested in good bokeh.
Aperture and subject distance are relevant as well. You might find that within a certain aperture range a lens has good bokeh, but it might get harsh when stopped down. The thing is, if you're using a camera where you can see what the lens is doing (i.e., an SLR or view camera), to pay attention to the out-of-focus area at the shooting aperture, just as much as you would pay attention to the subject.