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That's right, he didn't mention it, even that being one of the greatest mistakes in Linuxland.
And it's just not about making all programs look the same, but about usability, and I'll give a few examples:
1. Adding a side bookmark on a KDE Open/Save dialog box doesn't put that same bookmark in GTK dialog box and vice-versa
2. Gnome programs have their confirmation dialogs with switched buttons (the No/Yes question in Gnome dialogs)
3. Changing UI settings in KDE doesn't change them in GTK and vice-versa
4. It really looks bad
And these are just a few I remember now because there is probably much more.