I have been using Outlook 2003/XP for quite some time and am just
migrating to Entourage 2004/Mac OS.
There are tons of differences between 2 platforms. Just to show a few:
In Entourage, "project" concept is made to combine various things into
a single project. It is, in my opinion, a better "journal" than the
journaling function in Outlook.
Entourage doesn't use "display name" field. In Outlook, I typically
show person's name as "last name first name (company)" style. It is not
possible in Entourage.
Entourage can automatically "link" contacts and messages and events.
Outlook can also use "link" but Entourage's method is far more
automatic and painless.
Task and Notes are NOT synchronized with Exchange Server, if you use
Exchange Server. In addition, categories in Entourage is completely
separate from Outlook.
Latest version of Entourage utilizes Spotlight, and can also sync with
system's Address Book and iCal.
Since ActiveSync is not supported with Mac, PDAs/smartphone that uses
Windows Mobile platform cannot be used without third party software.