One of the best features is the data detection in the mouse pointer. Running your mouse over a date produces a small drop-down arrow that gives you the option to add it to iCal. The menu for addresses offers to find them in GoogleMaps, and names offer to add to Address Book, or show the Address Book entry, if one already exists.
The date feature can even recognize fuzzy dates, like ‘Wednesday after next’ and add them to iCal, complete with time, if that was included in the email. This effectively brings the link between Mail and iCal full circle, with the other half being reminder emails from iCal.
It’s got its flaws, sadly. For starters, it’s convinced that there is someone named “Rhode Island” who keeps emailing me and who desperately needs to be added to my address book. And, of course, it doesn’t do this in all parts of the OS. But the rest – it’s the little things that keep many Mac users using it, as opposed to the various other flavors of *nixy systems or even, gasp, Windows. This is a very elegant little thing.