There are many, actually, but here's one of them.
I get a normal email w/an attachment.
What's the first thing most people do? Open the attachment, of course!
Thing is..that takes you out of your groove and it takes a while to get back into it.
In Outlok 2007, you just click on the attachment and...
Voila...it shows up in the Outlook reading pane.
In seconds, you know what the attachment(s) is and you're off and running.
It's really nice.