The Yahoo mailing list server is notoriously unreliable and randomly drops mails and/or drops people from lists because their email server was temporarily refusing mails (4xx SMTP responses not 5xx). I also find the Yahoo groups web interface absolutely awful; mailman's list archives aren't great, but are ok for casual browsing, and you can download the entire archives and load them up locally if you need to.
A quick look around Google groups suggests that my initial assumption that it'd be as bad as Yahoo groups is probably unfounded. But if I hadn't already given in and created a Google account for other things, I'd be unhappy about doing so just for your list, given the way it tracks your web browsing in ways that I don't entirely understand while you are logged into it.
From "http://groups.google.com/support/bin/answer.py?answer=46438&topic=9244 ":
*Activities that don't require a Google Account: * - Reading posts in public groups - Searching for groups, posts, or authors - Posting to groups via email if they are unrestricted or you're already a member - Joining a public Google Group via email *Activities that require a Google Account: * - Creating and managing your own Google Group - Posting to groups via our web interface - Creating pages and uploading files - Subscribing to a Usenet newsgroup and receiving posts via email - Joining a Google Group via our web interface - Changing your subscription type (No Email, Abridged Email...) - Reading a restricted group's posts online Sean