Outlook on the web's new feature: Undo Send. It gives you a few seconds (up to 30 seconds) to change your mind.
A new calendar feature, called Interesting Calendars, is rolling out to Office 365 Exchange Online and Outlook.com users (on the new server). The current calendars offered are sports schedules.
I love conversation view, most of the time. It makes it easier for me to follow message threads as all previous messages in the thread are together, regardless of which folder each message is stored in. Not everyone likes it and finding where to disable it isn't necessarily intuitive.
How to change the destination folder of the one-click Archive button on the web version of Office 365 and Outlook.com.
A new feature for Office 365 email account is the Calendar Insights template for Excel. This is template creates a workbook connected to your Office 365 Exchange (or Outlook.com on the new server) calendar and analyses how much time you spend in meetings, the people you meet with the most, when your busiest time of the day is, and more.
How to change the colors used in Office 365 and (the new) Outlook.com's Outlook on the web.
How to open shared Office 365 Exchange mailboxes in Outlook on the web (OWA).