IE6 has long been the bane of the web design community. You design a site and it works in Firefox, it works in Opera — heck, it even works in Safari. But in IE6? Anything can happen, and, usually it does.
If you work in an environment filled with friendly, fascinating people, where you continually hear about exciting news (local, online, and around the world), and you are encouraged to be witty and playful, then you don’t need Twitter.