User-Agent: Opera/9.01 (X11; Linux i686; U; en)
Fluxbox and Pekwm implement a very efficient method of grouping windows, according to user preference: tabs. Using middle click, you drag a window's titlebar over another window's titlebar, and the two windows join. The resulting titlebar is split between the two, and you can use a custom shorcut to change between the two.
This method of grouping is more logical, and sometimes less disruptive than using an entire virtual desktop for some tasks. Example: Alt-Tab for changing between Gaim-chat-window<->Vim/Opera-group, and Win-Tab for changing between Vim<->Opera, in the same group (maybe for web dev work).
As both Fluxbox and Pekwm, which are light WM implement this, I don't think it might bring in great overhead.
PS: Also, the possibility to "undecorate" a window would be appreciated, but because it is possible by other means (devilspie, ...), it is not urgent).
Unfortunately I'm pretty much against grouping/tabbing, sorry. If that's what you want, why not using one of those WM that do that?
Because Xfwm integrates better in Xfce (obviously :) ), and other than that, Xfwm is a pretty powerfull WM (I'd say it's the most powerful WM so far in a DE, Metacity and Kwin are rather limited) :)
Let's say that it's not going to happen any time soon in xfwm4. I don't think being able to group different kind of apps is a natural way of doing things. Of course, when it makes sense, it's already done within the app itself, like Firefox or Terminal for example.
But do I would want to group Firefox and Terminal? I doubt it.
Moreover, I don't think it looks that good, visually speaking. It looks rather odd, IMHO.
Not counting the issues with apps' minimum/maximum window size which differs between apps, or even size increments, aspect ratio, etc. Too many cases of breakage (in the sense of window management, not in the sense of software bug).
I understand it's useful to some people, but my understanding is that the (few) people who cannot live w/out tabbing, cannot live without their favorite window manager, whatever window manager it might be. Of course, your mileage probably vary.