Well known, zero cost things that still are ignored too often.
Do them. On -any- platform, even mobile.
- Lack of self-occlusion. Pre-compute aperture cones on every mesh and bend the normalmap normals, change specular occlusion maps and roughness to fit the aperture cone. The only case where this doesn't apply is for animated models (i.e. characters), but even there baking in "t-pose" isn't silly (makes total sense for faces for example), maybe with some hand-authored adjustments.
- Non-premultiplied alpha.
- Wrong Alpha-key mipmaps computed via box (or regular image) filters.
- Specular aliasing (i.e. not using Toksvig or similar methods).
- Analytic, constant emission point/spot lights.
- Halos around DOF filters. Weight your samples! Maybe only on low-end mobile, if you just do a blur and blend, it might be understandable that you can't access the depth buffer to compute the weights during the blur...
- Cartoon-shading-like SSAO edges. Weight your samples! Even if for some reason you have to do SSAO over the final image (baaaad), at least color it, use some non-linear blending! Ah, and skew that f*cking SSAO "up", most light comes from sky or ceiling, skewing the filter upwards (shadows downwards) is more realistic than having them around objects. AND don't multiply it on top of the final shading! If you have to do so (because you don't have a full depth prepass) at least do some better blending than straight multiply!
- 2D Water ripples on meshes. This is the poster child of all the effects that can be done, but not quite right. Either you can do something -well enough- or -do not do it-. Tone it down! Find alternatives. Look at reference footage!
- Color channel clamping (after lighting), i.e. lack of tonemapping. Basic Reinhard is cheap, even on shaders on "current-gen" (if you're forced to output to a 8bit buffer... and don't care that alpha won't blend "right").
- Simple depth-based fog. At least have a ground! And change the fog based on sun dot view. Even if it's constant per frame, computed on the CPU.
If you can think of more that should go in the list, use the comments section!