One of the features that I miss the most from Microsoft Outlook is the email notifier that popups in the Windows task bar every time a new email arrives.
GrowlMail is a plug-in for Apple Mail that posts a Growl notification every time new mail arrives containing the text of the message - just like Outlook does on Windows.
Note: Installing Growl requires users to be able to follow directions carefully. It amuses me to no end to have a Mac user open a "Terminal" window and type Unix commands. Not what I would expect the typical Mac user to know how to do.