Middle-click to close windows from Activities overview
Something that has become part of my workflow across both elementary OS and Windows is opening their multitasking views for an overview of open windows, and middle-clicking (or three-finger tapping where supported) to close windows. This behavior is similar to how tabs work in both GTK and all major web browsers.
It appears middle-clicking is not distinct from clicking right now, so it just opens the window when the intent was the opposite—to close it.
I brought this up at the GNOME Parental Controls & Metered Data hackfest (on an unrelated tangent, as you do), and it seemed to be well-received.