I like the addition of stencil usage - it takes it beyond the obvious paint program idea and you can also collaborate with other people (via the interwebs potentially). This part of the explanation is key:
“The actual hardware tool of the artist – the spraying can – remains constant in its shape and function and is a catalyst for this software supporting innovative computer interaction. The self-explanatory program requires no previous knowledge or reference of a user’s manual.”
(Via Daring Fireball, which probably means you have all seen it already. Sigh).