I agree 100% with this. I have no idea why no company has developed a standalone word processor that just applies styles. Scrivener would be next to perfect if it just had styles -- and it's not impossible; the developer could implement it the way he implements footnotes. Word is next to useless because once you set up your style library, all the rest of the features never get used and tend to get in the way.

Anyway, there is one modern program that does exactly what you want: Adobe InDesign. It has a raw text "story editor" window that allows no formatting at all, only the application of styles. Applied paragraph style names are listed alongside your text on the left hand side. Other than that it's plain text... complete separation of content and presentation; you have to go back to the main InDesign window to set up your styles and format things. You can create and collapse footnotes in the story editor as well. The story editor even comes with built in custom looks to emulate green text on a black screen, or the amber text on blue of WordPerfect of yore. Unfortunately it doesn't use standard OS X text rendering routines, so you may have to futz around until you find a font that looks good... on LCD monitors I also prefer to select the "soft" anti-aliasing option in the style window.

As a bonus, InDesign generates great looking output (it has fantastic hyphenation and general typographic layout routines, including niceties like microtypography and margin kerning). InDesign, being intended to lay out books, is also extremely reliable for long documents (light years ahead of Word here) and complicated, multi-page tables.

There is also a team-based version of InDesign's story editor called "InCopy" designed for groups of writers working on a single publication. If you're only writing for yourself, it's not different enough from the built in story editor to warrant a look.