Re: Thank you!
> I like to see the window title on the taskbar - these might be quite long, which doesn't play nicely with a vertical taskbar.
This is not true, and it is a core point of my argument.
(I am also vaguely offended by the implication that I did not consider that.)
Taskbars are a Windows 95 innovation; there was nothing quite like it before. 1990s Windows has textual buttons in the taskbar. (Vista abandoned this, presumably in an effort to look more Mac-like.)
The essence of a taskbar is that when positioned vertically, the contents remain in horizontal orientation.
(Note: LXQt & MATE get this wrong.)
Xfce calls this a "deskbar" but it is the default behaviour in Windows. I would argue that there is no need for a separate mode or name. Most of the Windows 9x clone desktops get it wrong to a greater or a lesser degree, because their developers did not know how to properly use the thing they were attempting to copy.
When the taskbar is horizontal, window buttons take up a fixed width. If you make it thicker, then you get 2 rows, then 3, then 4, etc. (Note: KDE gets this wrong.) Only when the taskbar fills up do the buttons begin to shrink.
When a taskbar (or Xfce deskbar) is vertical, though, what is now its *width* can be adjusted, and this instead changes the width of the buttons.
(MATE, KDE & Cinnamon get this wrong, twice:  buttons get bigger, in all dimensions, not just wider;  status controls fail to be arranged in rows.)
So, if one sets the vertical taskbar *width* to the same size as the width of a normal horizontal taskbar button, you get exactly the same amount of textual information in it.
*However*, in vertical orientation on a relatively small screen, you can read the text labels on about 15-20 window buttons at once. On a horizontal taskbar with so many, now they shrink so much to fit that you can only see miniature icons.
It is thus desirable for multiple reasons:
1. You can have more open windows with readable buttons of the same size.
2. Window buttons do not need to shrink until you have many more of them.
3. You can afford a wider vertical taskbar than horizontal taskbar, because on a widescreen display, this does not consume scarce, hence valuable, vertical space, only cheap plentiful horizontal space. So you can see the same amount of a text file, or a document, or a web page, as on an old-fashioned 4:3 or even 16:10 display.
4. You do get less taskbar length in total, true, but with status icons in rows, you do not lose much panel space to them. (MATE, Cinnamon, Lxqt, Dash to panel, etc. all fail to handle this correctly.)
On a 4:3 display, the utility was marginal, but if you have 2 separate 4:3 monitors, as I routinely did from ~1996 onwards, or a widescreen display, as we all do now, it is very useful.
The hardware has changed. It is a good thing to learn to be flexible and adapt to this, and it is a bad thing to be rigid and refuse to adapt because one is used to the old ways. Take advantage of new facilities, including newer screen geometry.