I'm no expert, but my understanding is that the primary thing that causes bokeh-circles to be sharp is the (apparent) size of the light source. Smaller light-sources will cause sharper bokeh-circles.
To understand why that is, you must first understand what causes bokeh; please see this answer for a detailed explanation.
Once you've read that, it's easy to see why smaller light-sources will cause sharper circles.
The light emitted from a very small light-source. Note that the bokeh-circle is very sharp.
Imagine this as light emitted from two different points of the same light-source. Notice how the center of the bokeh is very bright, but the edges, where they don't overlap, are dimmer. When combined with the light emitted from all the other points on the light-source, this will cause the edges of the bokeh-circle to blur.
However, real-life lens have many more parts than the simple-lens from these diagrams, so there may be some factor I'm not aware of inside the lens that affects the bokeh-sharpness as well. And of course, a lens that is in-general sharper will also have sharper bokeh, simply due to being a closer-to-ideal lens.
Interestingly, the above also shows why brighter light-sources have more-opaque bokeh-circles: their spread-out light overpowers the spread-out light reflected off other nearby objects.