(Disclaimer: I am a Quakespasm user, with gl_nearest and square particles. My only real sin is that I don't turn model interpolation off).
It's not that hard to make DarkPlaces look fairly vanilla. Even the default settings in a fresh installation aren't that bad; once you've changed filtering to gl_nearest (which one would have to do in most engines anyway, I guess?) and turned off the blood stain effect, you're not that far from a vanilla aesthetic. Most of the garish bells and whistles aren't enabled by default.
This is what a default DP installation looks like where I've only changed gl_texturemode to gl_nearest_mipmap_linear" and adjusted the resolution to my monitor's; nothing else.
And using DP doesn't mean you're automatically using "high res" replacement content. Every instance where I've seen DP look "bad" (from an old-school purist point of view), it's been where the user has enabled all the optional extras and used replacement content.