* You could theme it however you wanted to an obscene amount. I had it display all messages right after each other in a small font without any linebreaks and I've never been able to have anything like that since then.
* The dock icon showed the names of the last few people who sent you unread messages
* It integrated with the OS X phone book app so you could it would display a single "John Smith" regardless of how many chat apps (AIM, MSN, Yahoo, etc.) you had them on
* It was actually smooth and not clunky (unlike Pidgin at the time and maybe half of apps today).