Google Chromes feature, whereby you can drag a tab into its own window, and then drag it back (or to another). I use this every day, and it hurts that I can't do it in other apps.
I used to think that window tabbing and grouping was too complex a UI problem for window managers to support. The implementations I'd seen required lots of keys and buttons to control the window groups, so it wasn't worth their time to implement something hardly anyone would use. But I was wrong, of course! There should be chrome-style tab grouping everywhere.