There are so many huge advancements in SharePoint 2007 that many of the little things that will make a huge difference are being lost in the shuffle. One of my favorites is the new people picker. The folks at Microsoft have figured out how to make the seemingly simple task of choosing a person in a web application as easy as choosing a person in Outlook. (…and NO I am not talking about the address book.)
Now when you add a user name to any field that is validated against Active Directory the entry is underlined just like in Outlook (and yes, you can force validation with [ctrl][k]).
Clicking the red underline provides options for replacement of the user selected.