“Over-filtering” is a broad term we use for technical and compositional problems that a photographer introduces into their image. Basically, it means that you've gone too far with some kind of editing or processing technique. The top candidates for over-filtering are over-sharpening, excessive combinations of sharpening and de-noise applications, and selective blurring to try and solve compression issues (skies are the most common). Selective blurring of people's faces (often done to try and avoid model releases), and sloppy editing of logos and banners can also lead to over-filtering.
We often commonly apply this rejection to mixed-media compositions of limited usefulness. Designers are looking for the raw materials for their projects – images that go too far mixing 3D renders and raster images, rasterized vectors added to other images, and overly-artificial raster manipulations, especially in background textures and wallpapers, can all be of limited use. Avoid making a finished design, and instead try to provide a designer with useful building blocks.
Particularly avoid weird plugins and filters that simulate water reflections, sun flares, rainbows, or any kind of artificial glow.
High ISO image with de-noise application and over-sharpening
Two examples of selective colour editing that look artificial. Supply either the original colour version or a well-converted black and white image only. The designer will do the rest.
Avoid any kind of glowing filters
All sorts of “cool” backgrounds and collages aren't useful as stock
4.0 Quality Standards
6.0 Title, Description & Keywords
8.0 Model & Property Releases