I'd be excited by further exchange / outlook integration to the point where *all* of the users settings were kept synchronized on the exchagne server. Everything from fonts, signatures, even down to the reading pane location and size could be stored centrally. Then when a user uses either another computer or OWA, these settings are all kept up to date to the latest settings they've made. This would save countless hours of reconfiguring outlook to users tastes when we have to replace laptops or setup roaming profiles etc.
And hopefully, by 2016, everyone will have adopted signed messaging, so we can be sure who sent what and when, with no legal ambiguities.
Also, I'd like to see in Outlook 2007 the Auto Archive feature to be disabled by default, rather than enabled. How many times has someone said (usually after their hard disk has failed), "Where have all my archive folders gone?"
And finally, a huge, huge thing that needs to be done - the autocomplete database for outlook that fills in email addresses as you type - this should be stored for each user on Exchange. Again, so that when they change between computers or owa, they don't lose this increasingly critical store of email addresses. RIght now you can't even move this database between outlooks, let alone recover it from a dead hard disk.