Lenses with straight edge aperture blades will give the cleanest, most sharply defined diffraction stars. The effect becomes stronger at smaller apertures (diffraction stars are an edge effect, and the ratio of edge length to area increases at small apertures).
Curved aperture blades will generate only weak diffraction stars since the curve causes the points to become spread out. As the aperture closes down the star burst improves as noted above, and also because the sides of the opening becomes relatively straighter as the curvature of each blade is covered by the next overlapping blade.
A perfectly round aperture won't produce diffraction stars at all, you will get a diffuse glow around any point sources of light (which could be considered a star burst with infinite number of points). That's also why you don't get star bursts when the lens is at full aperture.
An odd number of aperture blades will generate stars with twice as many points, producing delicate multi-pointed stars. For example a 7-blade aperture will generate 14-point stars. An even number of aperture blades will generate stars with only the same number of points but they will be stronger because the opposite pairs of blades produce overlapping points. For example a lens with 8 blades will produce strong 8-point stars.
