The most important things I see lacking in Kwin from KDE 4.5 are an API to allow windows to open in a specific existing group (make a new tab in the decoration), and that the windows from a specific group are not grouped in the taskbar. I also think that if these 2 problems are fixed, most apps in KDE could use the decoration tabs instead of relying on the currently used tabs, inside the application itself. The important thing to notice here is the natural mix between the application tabs and the menu button. These complete each other, and all apps in KDE which rely on tabs to show documents would ideally use this system (dolphin, rekonq, kate, kword… just to name a few).
Here are some screen shots proposed in the comment thread:
Having an easy API to enable this would be very welcome.
Elsewhere in KWin settings:
[ X ] Place tabs in window border when supported. [ ] Place main menu in window border when supported.