Except for the gui (by which I assume you mean wysiwyg) Emacs with Org-mode meets these requirements, albeit requires a bit of configuring and a couple add-ons. It takes a bit of learning, though, and wouldn't meet a user-friendly requirement. Org-mode does meet a few wysiwyg items (font size, underlining, italics, etc.) but certainly isn't full wysiwyg solution. Still, it's probably a better solution than LaTeX, because (1) Org documents can be seamlessly exported to LaTeX (and many other formats), (2) Org documents tend to have much cleaner (or even no markup) compared to LaTeX, and (3) add-ons extending functionality can be easily crafted in Emacs.