Outlook 2007 introduces a new feature that I personally find very useful: attachment previewers.  While reading a message, you can click on an attachment to get a preview of the file directly inside the message (without opening another application for Word, Excel, Images, etc.).  Following are a list of files supported out-of-the-box.

Microsoft Office Outlook 2007 items.
Microsoft Office Word 2007 documents.
Microsoft Office PowerPoint 2007 presentations.
Microsoft Office Excel 2007 worksheets.
Microsoft Office Visio 2007 drawings.
Images and text files.

Combine this with flagging and the reading pane which I already used, and I can’t really think of an email management tool on the market that gets anywhere close to Outlook 2007’s efficiency. 

How many times does someone send you pictures that you would love to just quickly view rather than opening them in an image-viewer?  Here is a picture of a .jpg previewer in action.

Outlook 2007 Previewers