Apple has posted a video guided tour of the upcoming version of OS X, Leopard.
It seems rather cool.
It seems that the "big secret feature" that Steve Jobs hinted at many moons ago has gone awol, though. That's probably why it was secret: they were not sure it could be done.
The new iChat includes screen sharing. If the other person accepts, you can see their screen as if it were your own, and you can even work with documents on it, while you're chatting about what you're doing. Quite impressive. This feature has existed in other kinds of software, but I bet iChat makes it easy.
Perhaps the most significant feature, though, is Time Machine. Only a very tiny percentage of users do proper backups, or even any backups at all. Time Machine is an easy and fully automatic way to make sure this is happening.
(I wonder why Apple has decreed it will only work with an external hard drive, though. I'd like to make use of the nice four drive slots that my Mac Pro has.)
Update: For help with Mac upgrades and many other technical issues, I recommend TidBITS Take Control books, many of them written by Joe Kissell, who is very helpful both in his books and personally.