GNOME has enough weight to at least force most projects to accommodate them. But unfortunately this has mostly been for the worst, as GNOME is usually the odd one out with most matters of taste and design.

Maybe to some degree that's true. But let's take an example: GNOME is the only (afaik) desktop that requires client-side decorations. They've been like that for years, but nobody else is following them on that. Yes, the toolkits and a number of toolkit-less apps have added support for them. But it's not like they were actually able to employ their gravity to change the world over to CSD (thank goodness).