I've been wanting to make some improved Windows 10 (and maybe others) customization tutorials for a while... For now, I've just made a list of a few faults I found that can likely be improved in most tutorials I've seen:

Windows 10/11 to

XP - Terrible Start menu in everything I have seen, terrible theme in some tutorials, weird Explorer UI that could (probably?) look better, icons (use IconPackager or modify icon .mun files), no 7+ fallback boot screen that can actually be changed to look exactly like XP’s (use the tutorial from Nobel Tech https://www.youtube.com/watch?v=mZBHz2x5IV0), no Tahoma/Trebuchet MS fonts or ClearType disabling...

Vista - Bit of a better Start menu that could be used (look around on Classic Shell forums), nothing for enabling the 7+ Vista-styled fallback boot screen, icons (use IconPackager or modify icon .mun files), no actual window theme most of the time (I found one or two with a good WindowBlinds theme, but acrylic blur was not disabled and had an awful navbar)…

7 - Weird, bizarre 10 boot screen logo replacement rather than the fallback Vista-styled one in 7+, acrylic blur kept on in WindowBlinds (really good otherwise from Windows Saturn, http://web.archive.org/web/20230121212255/https://www.youtube.com/watch?v=91i1F3yzLa0)

I'll probably add to this as I find more things to point out.

UPDATE:
Windows customization of the latest versions has evolved a lot since this post. I feel theme-patching and using themes like Aero10 along with tools like DWMBlurGlass negates the use of WindowBlinds for theming 10/11 to Vista/7 now as they look better and more accurate than the WindowBlinds themes (plus, WindowBlinds is a bit buggy and doesn't run the best on some machines...).
Mods such as Windows eXPerience have also shown the great customization of 10/11 to XP when enough effort is given (also with theme-patching and some extra tools).