I’ve tried several of the minimalist word processors out there. For Windows only, I’d say Q10 is my favorite — clean, simple, and fast. However, I’ve been on the go a lot and find myself using all three major platforms (Windows, MacOS, and Linux) during the course of a typical day. If I want to sneak in a bit of writing at any moment, a cross-platform solution has turned into a must. I also realized that I do miss certain features that many “minimalist” word processors might consider extravagant.
I’ve fallen in love with FocusWriter for drafting new stories. It is not only cross-platform, but comes with a greater degree of control over the writing environment, without sacrificing the simple and clean approach to the writing itself. Put a different way, it’s actually not minimalist — it has everything that a fiction writer would want, and simply nothing more. Features: full-screen mode, instant word count, daily targets, session timers, scene dividers, and customizable themes.
Below is a screenshot of the custom theme I created to work on my Camp Nanowrimo novel, working title Red Soil Through His Fingers. Since the novel takes place on a Mars colony, the writing environment lends to the mood. (Don’t judge the writing sample too harshly… its a first draft idea dump…)

The background is a shot from one of the Mars orbiters, with blue-grey Georgia 16pt on 85% opacity black in the center. Ping me if you want the theme file.