There are three levels of nested fieldsets. IMO there should only ever be one level of fieldsets, ever. Distinct stuff could just have it's own fieldset, or none.

This raises the issue between content type and global settings - the fieldsets are handy for that distinction, but there's so many levels within each that I think that distinction is lost (or at least I didn't get it quickly). The way Views 2 sets up "defaults" first - then allows different displays to override these could probably be copied almost wholesale. This would be really easy conceptually if "Fields" moves to the top level in site building, then you add a field before attaching to a content type - then you could add the defaults + global stuff together, then use the "stack" stuff to cycle through override settings for each field instance.

Or just some other way to indicate the distinction which isn't fieldsets, will have a think.