A lot of good work done there Martin. Nice job.
I'd like to add a few comments (some of which I might be repeating from other posts).
I think the amount of "stuff" in any panel - for example, "Properties" - is important and can greatly affect how software is perceived and used. More specifically, the more scrolling the user has to do to find things in a panel the less professional it feels to the user.
To someone looking at the software for the first time it might look like there's loads of options for them to play with but in actual use it becomes tiresome to have to keep scrolling up and down to get where you want.
For example, in the Properties/Text tab, if you have a few of the sub-tabs open - unless you have a very large monitor - you have to scroll up and down to find the thing you want, e.g. Colour, Style, Columns, Advanced, etc. (See the image I posted earlier in this thread.) This can also have the effect of the user inadvertently making changes with a scroll wheel if the pointer happens to be over a control that accepts scroll wheel input. (Also, this sort of inadvertent change can be difficult to track down if you weren't watching.)
It is better to keep scrolling within panels to a minimum. By way of two examples, if you look at GIMP and Blender you will see that their controls are arranged in such a way that the user - even on smaller screens - has very little scrolling to do no matter where they are in the control panels. There are more panels to click between but they are each small enough to fit on the screen without scrolling. This can give the user a much better experience.
2. Content/Frame Split
I would be generally in favour of splitting content properties from frame properties as long as it was done well. (Even now, after using Scribus for a few years, I still find myself - every now and again - going to the Colours tab to change the colour of text. Stupid I know but my brain thinks "colour" and automatically moves my hand to point to the "colour" tab which is always visible.)
My main concern with this would be implementation. For instance, "XYZ, Shape, Group, Line and Colours" are all frame properties while "Text and Image" are content properties. So how would the two things be split? How would putting them in separate tabs in the same panel affect the UI? Would there be two separate panels instead? How would that affect screen space if they were side-by-side? If they were one-above-the-other that might cause major scrolling issues. Would a context-sensitive toolbar - as in Inkscape - be a good option instead? This would need a lot of trial and error which - I'm guessing - the developers won't have time for.
On a related note: How easy would it be for someone to put some kind of non-functional properties UI together that people could mess around with in Qt? I've no idea how much time this would take but it could help more people be involved and might allow for a quicker solution in the end.
I think the road Martin is going down is correct with a "compression" of the controls within the panels. At the moment, Scribus is quite "newbie-friendly" with lots of space and long control names/descriptions but it makes for a lot of unused screen real estate (and tool-tips are usually available to give more explanation).
For example, in the "Shape" tab, the "Disabled, Use Frame Shape, Use Bounding Box, Use Contour Line, Use Image Clip Path" options could all be compressed into the space of just the first two by putting them in two columns and omitting the un-necessary wording, e.g. "Disabled, Frame, Bounds, Contour, Clip Path".
For an example of a good lean UI I always point to the GIMP Toolbox. The toolbox and toolbox options give the user a whole lot of control but manage to fit all of that into two panels which can sit unobtrusively to the side of the main window. I haven't counted but I'm fairly sure that GIMP puts way more tools and options into one small double-panel - which seldom needs to scroll even on smaller screens - than Scribus puts in one panel that needs a lot of scrolling.
I realise that a toolbox isn't the same as a properties panel but the same sort of layout design could be used.
Getting a nice UI that is both streamlined and easy to use is difficult but any improvement would be good.