Move key action buttons (SEND, EDIT MEETING, CANCEL, ...) to the top of the windows
The key actions in views (like Reply to email, View meeting / event) like SEND, DISCARD, EDIT, CANCEL, ...) are placed at the bottom of the windows, not in the toolbar at the top.
This might be good for a mobile app (closer to your thumbs), but in a desktop app:
They get lost: especially if you're replying to a "busy" HTML email with lots of images and colour, the buttons at the bottom can visually blend into the body of the email and be lost
They're not where your mouse is: typically you're adding a reply to the top of an email, or editing a time/date/location in an event. Your mouse is at the top of the view, and then you have to find the button at the bottom, and move the mouse down. It's slower, distracting, and can be a bit confusing.
The normal context-sensitive toolbar/ribbons in Office are great because relevant actions are right there where your mouse is most likely to be - or at least in a consistent location with other actions.