Hmmm - one major issues and one personal dislike:
1. Notes & Tasks: If you want to add or view tasks and notes in the new Outlook view, it will open your default web browser, you must log into your MS account and add notes and tasks through the web interface rather than directly into Outlook. This is specifically annoying if you have multiple accounts in Outlook. In the classic version, notes and tasks are associated with any of your active mail accounts you choose - directly in Outlook. Now you have to get out of Outlook, into the web interface, log in with the account you want the note or task associated with and add it in the web interface. Who the hell missed this peeps?
2. The design: Obviously design is a question of taste and personal preferences. While I see the effort....it just looks more like something geared towards a teenage girl than a professional user. Just my opinion.