Well, the answer is quiet simple and here are a couple of reasons:
Conversation view: Nested view of each conversation, with the ability to see both incoming and sent items in one thread without having to change to the sent folder. This is also available in Outlook Web App which is great.
Multiple Exchange accounts: It is possible to have up to three Exchange accounts in one Outlook profile. No more creating separate profiles for multiple Exchange accounts.
Ignore conversation: Are you familiar with being spammed with a lot of unnecessary emails, like “where will we eat lunch today”, “watch this funny movie” etc etc? With ignore conversation all future replies to such a conversation is automatically directed to the Deleted items in Outlook. Sweet!
Mailtips: When about to send an email to someone who is out of office you will be notified by this instantly before sending the email. This is by default but you can also create your own mailtips rules for certain distribution groups etc.
I must also mention improvements to IMAP which is splendid! So when will Office 2010 go to RTM?