It's a nitpick, but X11 isn't a window manager; it's just a graphics layer - more like OS X's Quartz. If you run Gnome Shell with compositing enabled, your window manager is Mutter; if you run Gnome Shell in fallback mode, your window manager is Metacity. That is, unless you change it, and that is a reason I love Linux.
Very true, I misspoke, and meant more in the terms of "standard X11-based window managers", which all tend to share the same basic set of features, eg, highlight/middle-paste, alt-dragging to move windows, etc.
