I think I was still using GNOME 2 when I wrote it, at least on my desktop, although in any case Alt+Tab wasn't very usable in Xfce until 4.12 (the current release, from 2015).
As far as I'm concerned, a display of large icons as has been standard since at least Windows 3, is virtually useless. "Oh look, I've opened five text documents, ten PDFs and five file manager windows. I totally know which is which..." Mini thumbnails don't help either. "Oh look, all of my text documents have text in them. I can totally tell them apart now." When you find yourself using the much more awkward taskbar instead of Alt+Tab, you know Alt+Tab is broken.
A usable window list is structured like a text-based list consisting of icon + window title. That's the bare minimum. Xfce has that option nowadays, so it's alright. Unfortunately it's mildly hampered by some stupid transparency that can't (easily) be disabled. But like I said, that's just the bare minimum.
The brilliance of SmartTab.org over the traditional window switcher (which of course I shamelessly copied), besides the list-based approach, is to assign a quick way to access each window. So in nimbler, I activate the window switcher (my nimbler shortcut is Super + `), I identify what I want to switch to, and then I press, e.g.,
c. Another thing is that you can use the arrow keys or the mouse to activate a window as opposed to having to perform Alt-Tab contortionism.
Something I copied from superswitcher is to show all windows on all workspaces, organized by workspace, so I don't have to switch to workspace 3 first in order to go Alt-Tabbing around workspace 3. You can also quickly switch to a workspace using F1-F12, although that's fairly redundant. In superswitcher you can also create new workspaces and drag windows around between workspaces so it's more useful there.

On the "someday" list is to add a function to highlight windows matching a search term similar to superswitcher. You can already activate the GUI design for this by pressing colon (:) but it's otherwise useless.