I always feel that tabbing shouldn't be part of the application, it should be part of the window manager: it's not clear that it benefits one application more than another.

For one thing, applications are the wrong conceptual groupings. I want the editor windows, compiler windows, Wikipedia look-up and e-mail thread for a problem I'm working on grouped together, rather than all my editor windows in one place, PuTTY windows in another, etc.

For another, I seem to know better than application designers whether I want virtual desktops, MDI, tabs, tree tabs or separate windows in any particular circumstance.