As with the Dock review they did last week, they first take a look at the history of iChat, but iChat actually got started fairly late in the world of instant messaging. Apple started it out by working with other protocols (one reason it's worked so well since then, I'd bet). iChat AV was the most significant update so far, adding videoconferencing that, thanks to the built-in iSights on most Macs nowadays, made the feature widespread.
To me, iChat 4.0's most exciting new feature is the document and screen sharing. Via VNC, both Mac and PC users have been able to share screens for a while, but the process has always been complicated (in most cases, so complicated that the problem you're actually trying to fix by screen sharing is less complicated than sharing screens itself). But iChat 4.0 is meant to make the process easy. There are also lots of fun video implementations-- including an "Obi-wan" setting I can't wait to try-- to go along with videoconferencing.
Day to day, I generally use Adium as an IM client (the look and configurability sells it for me), but the new features in iChat 4.0 will definitely tempt me to check them out, if not switch completely. Just 9 days left!