I am a moderator of a few private eGroups and the transition to Yahoogroups was mostly painless, if you followed some simple steps. The Yahoo interface has additional new features over eGroups, including message preview.
Members can take polls, post pictures in files areas.
There a very cool excel like database feature for scope inventories, etc. A Links area to post useful links. Member roster, editable profile options. And most importantly, for high volume lists like TAC is becoming, options for Members to select Web-based only reading, individual emails, or a daily digest. For those who don't like cookies, there is a cookie-less operation scenario I've devised.