For once, I'm glad I've horded months worth of spam in a junk folder: it's the perfect raw material for training Baysian filters, which the new version of Mail comes with.
I haven't tried out iCal yet, but the calendaring software was one of my main incentives to invest in this particular upgrade, so I have high hopes. We'll see.
I installed the full set of language fonts which came with the OS, including the optionals, like Cyrillic, Arabic, and Hebrew. I don't know how any of them are integrated yet, but it'll be fun to play around with them. Of course, it also installed the default language scripts, such as Finnish and Brazilian Portuguese.