Dan Moren at MacUser just removed a long-standing headache of mine and I'm sure many others by posting on the ability to specify default addresses for Address Book groups. Here's a little background if you're scratching your head: in Mail.app, you can type the name of a group to address a message to every member of that group. The catch is that if any of those members have more than one email addresses in their card, Mail.app arbitrarily chooses which address gets used when that member added as part of a group. This can cause all sorts of problems, ranging from minor annoyances all the way up to sending a NSFW (Not Safe For Work) forward to exactly the wrong address.
Enter the wonders of being able to set a default address for these members in groups. This removes the worry from using the power of groups to address your messages. All you need to do is select Edit > Edit Distribution List... (not the greatest name for this feature, IMHO) to access a panel which allows you to chose default addresses for all the members of your groups. What's even better is that this panel allows you to simply specify one label or another (Work, Home, etc.) for all the members in that group, instead of forcing you to manually chose labels for each member, one by one. Nice.