In a blog post from 2004, Swartz writes:
Together we pored over the syntax details from top to bottom, trying to develop the perfect format, and I think we've got something pretty darn great. We've tested it extensively: on our blogs, in my comments form, in our emails. It's all worked amazingly well.
The format, and John Gruber's perl implementation of it, is called Markdown. I highly recommend it. It plugs right into Movable Type and makes writing entries so much easier and fun.
Swartz was right on: Markdown is easier to write in, fun, and pretty darn great. I can't imagine ever writing in a bloated word processor ever again; plain text and MD are where it's at for me.
Also, consider that Swartz was only 26 when he died a couple days ago. That means he was only 17 when he was tapped by Gruber to be the lone beta tester for the project. Crazy.