I remember the (text) group chat in ToME was pretty cool when I first started playing that. (4 years ago... holy crap) There were folks on who welcomed me to the game, commiserated with my first death very shortly thereafter, gave me some tips when I asked for advice, etc. It really made me feel welcome and less lost trying to figure the game out for the first time.
It should definitely be easy to turn off, have an easy "ignore" function (for that one guy who never shuts up/trolls), and have both a general room and optional side-rooms and user-created group chats for those who just want to see what their buddies are saying, not the entire world.
Having a colored message pop up in the world chat whenever a user on it earns an achievement would be cool too.