Tabs are a simple, intuitive GUI enhancement that decades after introduction are still being re-implemented by hand in different ways by the minority of applications that support them, including multiple XFCE applications. A window manager that offers native tabbing support not only automatically gives tabs to ALL applications, but it could allow for a much greater customization for how the tabs are manipulated and presented.  This might include putting them on the title bar, like many browsers do in Windows, but it could also put them below the title bar or perhaps even on a separate "tab bar" that sits above or next to the the task bar (i.e. "Window Buttons").  Nothing needs to break; applications that already implement tabs could be set to use those instead, although an eventual long term goal might be to create a standardized interface by which applications could talk to the WM-implemented tab manager. 

In other words, although the initial idea should be pretty straightforward to implement (one process per tab, tabs appear below the title bar, keyboard shortcuts and mouse commands to manipulate tabs, disable WM tabs for applications that have their own tabs), there is potential for lots of further improvement and innovation, both behind the scenes and user-facing.

And this is all without being disruptive to anyone's work flow.  *Everyone* understands tabs. And you either use them or you don't. But if you don't use them, they don't get in the way. It's not a radical new paradigm you have to work to sell anyone on. This stands in marked contrast to most of the other "innovations" attempted by certain over unnamed desktop environments over the last decade.

The basic concept has implemented at least once in Windows (windowtabs.com), but I have yet to find any desktop Linux project that attempts it. The simple, lightweight, intuitive usefulness of window manager driven tabs seems like it should be right up XFCE's alley.  And it's the sort of thing that would instantly make thousands of power users (not to be confused with experts, who by and large are happier on a CLI) sit up and take note.