I see the discussion as wobbling off its axis already, much as the slipstream discussion did over the course of time. Ryman's original definition (mentioned in the Wiki link above) seems concerned with a specific set of constraints defining what I would call a sub-genre and what others might call a movement. (Note there's a meaningful difference between those two terms — "sub-genre" is a post facto marketing category, "movement" is a self-conscious direction set by writers themselves, presumably pre facto.)
However, the discussion seems to be wobbling into the separate discussion of entry-level SF. As I see it, much of mundane SF (pace Ryman) would serve well as entry-level SF, as it is stripped of many of the more explicitly exotic tropes of our field, while providing the inexperienced reader with the assistance of a significant naturalistic component to the work. That is a somewhat coincidental and almost certainly inaccurate overlap.
I also find myself wondering if there is mundane Fantasy, by extrapolation. For example, does paranormal romance, such as Carrie Vaughn's Kitty books, fall here? They are largely set in the naturalistic world with a relatively isolated set of variances for the purposes of the story.
There's something about the discussion of sub-genres and movements which seems to inflame us all. Ryman's definition is pretty easy to interpret, simply by taking him at his word. But a set of stylistic choices and story constraints is one thing. Solving the Gordian knot of entry-level SF is another. Part of the allure of Mundane SF is that it hints at opening a door many of us would like to see thrown wide.
I just don't think it's the answer.