I don’t think it is currently possible to opt out of the default styling, yes it can be overridden but that would add more work than needed. maybe make inserting the default styles lazily on construction?
Part of the default styling is necessary to make the editor work at all. Which styles did you want to opt out of, specifically?
I for one am using a custom menu that’s embedded in the page and I believe managing the other styles myself would be preferable.
So in answer… all of them 
- and maybe making classes that get toggled (like for node selection) configurable. ProseMirror-selectednode is gross (no offense), I’d prefer is-selected. So why not allow users to have control of this stuff? Allows us to use whatever naming/styling patterns we want.
You don’t get any menu-related styles unless you include menu modules.
As for the CSS class names, you’re going to have to just deal with them.
Most of it … maybe allow opting out of visual styling and keep transitions?
+1.
As far as I can tell there’s no way to opt-out of the default tooltip styles (.ProseMirror-tooltip) which are inserted from ui/tooltip.js.
If you don’t load the tooltip addon, you don’t get those styles. If you do, you’ll need some of them, and can override the ones you don’t like.