I personally think that having a Meego Touch preview release would help with this, as quite a lot of people are having problems setting up the entire stack, as evidenced by that mailing list thread and some of the emails I'm getting; and ofc my own experience).

For example I am yet to make the compositor work (but tbh I've not yet put much thought into it as I've not really required it so far). But quite a lot of interesting libhildon stuff is the close integration it had with the fremantle window manager / compositor, and that is going to be hard to port to MeegoTouch.

One trait of MeegoTouch is that the compositor seems to be way less involved than the Fremantle compositor ever was, seeing that I can run MeegoTouch samples under both KWin and Metacity and the title bar is shown! *

In fact, from what I gather, each MeegoTouch application seems to be designed to use as few X windows as possible: menus are drawn inside the parent window, "stackable windows" are all handled by Qt on a single X window, etc. **
Hildon used quite an opposite concept, creating an X window for every stackable window, menu, tooltip, ... and letting the compositor take care of it.
So there's going to be quite a lot of work to be done at client level (inside libhildon or -- god forbid -- inside more libgtk patches) that was previously done at server/compositor level, like arranging the menu windows.

And that doesn't look nice. Does Gtk+ have something akin to QGraphicsView?

There's still something I'd like to know: how does mcompositor interact with "old style" windows? Plain Qt applications? What will it do when an application opens a menu X window?

*This also answered one of the first questions I made -- if titlebars are transparent, how does the application know it?
Answer -- applications draw their own titlebars. So they of course know
** Rant about "diverging from desktop Linux" goes here. In fact the only way I managed to get DUI to create an extra X window is by showin a System Model window.
This entire behaviour might be caused because I'm not running duicompositor, though.