X11 and X.org are two different concepts. While an overwhelming majority of questions are about X11 as implemented by X.org, there are a few questions about other implementations. Furthermore X.org may one day no longer be the de facto standard X11 implementation (remember XFree86?), so I don't think we should lock ourselves to it.
I'm aware that X.org's overwhelming domination makes this difficult, but we should strive to use x11 on questions about X11 in general and xorg on questions about the X.org implementation specifically. This is similar to the distinction between unix (which is implicit on all questions on this site) and linux, or between shell and bash.
- How do I minimize a window from the command line? x11
- How do I make my graphics card work? xorg
- Keyboard remapping x11
- OpenGL acceleration xorg
There are undoubtedly many gray areas.
There are surely a lot of questions with the wrong tag right now, but we should try to get it right most of the time.