Outlook 2010 builds on a number of features in Outlook 2007 and adds several popular requests, including the ability to open Exchange mailboxes on two different Exchange servers.
The big change in this version is the ribbonized interface, with everything that used to be on the Tools menu now on the File tab (also called Backstage). (Some of the options are exposed on the ribbon too.)
One change that makes sense but goes against 10 years of usage is the location of the Import and Export commands. Import/Export is now on the Open command.
Support for two (or more) Exchange mailboxes. This works best when the mailboxes are in different organizations, but you can use mailboxes on the same server. Note that when you add multiple mailboxes on the same server, if public folders are available, they will be added multiple times.
The From account for new messages default to the mailbox you are working in. Sent items go into the correct account's sent folder.
Rather than using an NK2 file for the nickname cache, the cache is stored as a hidden file in the mailbox (and moves with you) and contacts are created for addresses you send it. These are stored in a folder named Suggested Contacts. You can clear the nickname cache from File tab, Options, Mail. Empty AutoComplete button is halfway down the page.
Conversation view: messages are grouped by conversation. It doesn't always work as expected though and many users don't like it. To disable it, right click on the row of field names and choose Conversations, Show in Conversations to toggle it off and on. You can also change the setting from the View Ribbon, Conversations button.
All day events are "pinned" to the day, not to the time. This means changing time zones will not make all day events spread over 2 days.
Calendar colors can be set for each calendar. To change the calendar color, right click on a Calendar and choose Color.
Calendars can be arranged in groups and the group enabled or disabled by clicking in the box next to the group name. (Calendars can be in more than one group.)
Autocomplete list / Nickname cache / NK2: The NK2 file is no more. The autocomplete cache is stored as a hidden file in the mailbox or PST and addresses you send messages to that not in your Contacts are added to the Suggested Contacts folder for safekeeping.
To clear the nickname cache: From the Options dialog, go to Mail, Send messages section (halfway down) and click the button to clear the cache.
If your autocomplete list becomes corrupt, Outlook 2010 includes a safeguard: Suggested Contacts. Addresses you send mail to (new mail or replies) that do not already exist in your contacts while have a contact created in Suggested Contacts. This can be disabled in File, Options, Contacts if you don't want these contacts created. For more information, see Suggested Contacts and Auto-Complete Lists
Tip: Look on the File tab for Options and Account settings.