If you want to see what all the fun is about, I recommend perusing some of the logfiles, that repository of every word written in and to the list. I keep a (mostly up-to-date) set here, as part of the Mythus web pages.
That's all well and good, but how do I join?
Send e-mail to firstname.lastname@example.org, with no subject. The body of your message should contain:
SUBSCRIBE MYTHUS-L Your Name
Of course, you need to replace Your Name with your own monikor (e.g. Gary Gygax or Dave Newton, if that's who you happen to be!).
If you have problems joining, I can probably help, so feel free to e-mail me.