Comment 15 for bug 1212821

Ok, well, it is trying to use llvmpipe. A fairly recent change in Nux handles this now (http://bazaar.launchpad.net/~unity-team/nux/trunk/revision/810#debian/50_check_unity_support).

Looking at the gnome-session.log, the failure is occurring when the open plugin is asking if the X session supports OpenGL/GLX and it is returning false which causes the opengl plugin to fatally error out. From the opengl plugin:
    glXGetConfig (dpy, visinfo, GLX_USE_GL, &value);
    if (!value)
    {
        compLogMessage ("opengl", CompLogLevelFatal,
                        "Root visual is not a GL visual");
        XFree (visinfo);
        screen->handleCompizEvent ("opengl", "fatal_fallback", o);
        setFailed ();
        return;
    }

I suspect something in llvmpipe is misbehaving, but what it could be is beyond my knowledge.