Although I haven't installed the latest Beta 1 refresh of Outlook 2007 yet, I've already decided what my favorite feature of Outlook 2007 is.  Hands down, its the To-Do Bar.  Outlook has had task integration for the longest time but it just hasn't been seamless for me (and I suspect, a lot of other people) and the end result is people use Mail and Calendaring a lot, but not much else.  The To-Do Bar was the missing piece that integrates everything nicely.  You can get lots of details from Melissa's blog here:

http://blogs.msdn.com/melissamacbeth/archive/2005/12/20/506047.aspx

For the latest updates in Beta 1 refresh, check out this later posting here:

http://blogs.msdn.com/melissamacbeth/archive/2006/03/16/553227.aspx

If you don't know what I'm talking about, here's what the To-Do Bar looks like:

It ties all the important elements of Outlook together like the Today page was never able to do.  It forces you to be proactive and informed about what's coming up all at once.  It also allows for easy prioritization and "quick view" of your next tasks immediately.  I know it may sound like a small detail, but the productivity boost for Information workers is awesome and readily apparent when you start using it.  Trust me, you'll love it!