What is a conceptual font? For me it's a font that exists because of a strong idea. Some examples:
Westminster: Machine-readable numbers (MICR) adapted to the Latin alphabet.
Helvetica Now: Helvetica recreated for today's technology.
Comic Neue: The font that saved Comic Sans.
What I love about them is they market themselves. They're something people talk about. They're viral.
What other examples are there?