• "Request in-person" meeting option - this option allows you to select the option to make the meeting in-person instead of online
• The ability to pin important emails to the top of your inbox – you can pin important emails you don't want to forget about
• Limited support for 3rd party email and calendars such as Gmail.
• Autoreplies can now include having your calendar blocked off and cancelling meetings you have scheduled during your time away.  See Setting Up Auto Replies and Out of Office Messages
• Shared account calendar – you can remove a shared calendar from your calendar list if you only use the shared account for email
• Search results show which folder the message appears in
• Undo send option for up to a 10 second delay before sending a message.  Allows you time to stop it from going out and re-thinking what to send.  Settings (gear icon) | Mail | Compose and Reply | Undo Send – slide bar for amount of delay
• More Settings options – you can now setup and configure the following in Outlook “New”
  • Settings | Storage - gives a breakdown of mailbox folders and amount of space used
  • Can set Microsoft 365 email forwarding
  • Can share and publish calendars (used to have to be done in OWA)
  • Bookings with me is configurable in Outlook “New”
  • Work hours and locations feature
• "Edit this and all following events in a series" option 

• Shared Accounts – if you access or manage a shared departmental account, you will see changes.
  • When you switch to the “new” version, some (or all) of your shared accounts may not transfer over and you will have to re-add them individually.  In Mail, right click on your email address in the folder pane.  Select Add shared folder or mailbox, type the account name or email address and click Add
  • For some the following may be missing from shared account functions
    • Shared account Inboxes don't automatically update the unread count or show you have new mail -- you have to click the mailbox in the Favorites to see any new items
• View settings apply to all folders - you can't set one different from another.  For instance, if you want one folder to have the reading pane on but not all, you can no longer do that.
• Signatures – if you use roaming signatures, those do not appear in the “new” Outlook.  You may have to add them all manually.  Check Settings (gear icon) | Accounts | Signatures – to see if your signatures transferred.
• Messages that have been opened don’t automatically change to "Read". You may need to change your settings for how that feature works – Settings (gear icon) | Mail | Message handling – Mark as read section
• ToDo bar is not available – Use My Day instead. Click the My Day icon in upper right corner of Outlook window.
• Rules are missing options – the number of actions for rules is less than what Outlook for Windows (Classic) offers.  You are now limited to these actions to take on a message:  

• PST files are not fully supported - .pst files can be opened but view only access at this time
• Import/Export option - limited options
• If you use color coding with the View feature, that is not available.  Assigning a category will now show in the message list in the Subject field but the whole message will not have the category color
• The new interface has the look and feel of Outlook on the web (OWA)
• No Address Book icon to lookup users
• No Folders View using the Folders icon
• User opens multiple calendars, they don't fit on one screen, you have to scroll to see all of them
• In Calendar, no way to view calendar in different ways -- by list, preview, active, calendar etc.  You can't manage the views either
• View menu missing -- can't change views or settings for the current view to apply color coding, settings, etc.
• Copy feature is no longer available.  You can't copy a message or folder and paste it somewhere else
• When scheduling a resource and using the "Add a room with Copilot" feature, if you select "Rebook declined rooms" - please note that Outlook will continue to send messages everyday until it finds a replacement room if there is a conflict in your recurrence series.  Even if you delete the meeting series, it may continue to search for a room and send you notices. 

For more information, see the following articles:

Getting started with the new Outlook for Windows

Feature comparison between new Outlook and classic Outlook 

Youtube videos for new Outlook

Microsoft to support classic Outlook for Windows until at least 2029

Starting to support third-party accounts in New Outlook for Windows Preview