Entourage vs Outlook

In brief, Entourage has many of Outlook's features. Here are some major features that are missing:

• cannot edit server-based global distribution lists
• cannot access nor edit personal distribution lists stored on the server
• voting buttons are not supported
• cannot edit server-based rules
• cannot sync tasks and notes with the server
• cannot easily request delivery and read receipts, see this page for a workaround: http://www.entourage.mvps.org/faq_topic/return_receipt.html
  

Here's a much more complete list: Microsoft's Mactopia article on Entourage vs Outlook