I generally prefer to live with defaults, so the lack of theming and user interface customization in Gnome is a good thing in my opinion. I tend to take the view that software that requires too much customization is poorly suited to my needs.

I suspect the thing that rubs a lot of people the wrong way with Gnome isn't so much the lack of customization or theming, but actual decisions behind how the user interface should work. People simply complain about the inability to customize Gnome as a reflection of how powerless they are to do anything about those decisions.