To add to the realism of the sketchy lines, we made each slightly transparent to make the lines seem more realistic. This added a number of depth-buffer-based transparency problems, but were alleviated by disabling the depth buffer entirely.
The particles are drawn as small jittering crosses.
As the creators explain:
Non-PhotoRealistic Rendering (npr) is often considered the task of taking a 3d environment and displaying it as if it were (for example) hand-drawn. Stylization is the main purpose of an npr, as is with most pencil-and-paper artists (imagine if everyone who drew cartoons tried to make them look as realistic as possible - boring!). NPRQuake attempts to capture the elements of different drawing techniques, and immerse the viewer in worlds drawn entirely in the prescribed style. If you have ever imagined running around inside a painting or a drawing, you are beginning to get the idea.