Layer styles are useful because of their parametric and their non-destructive natures. Smart Filters are useful, but require the creation of a smart object beforehand.
I'd like to suggest new layer styles, of commonly used effects, that would help to automate time-consuming tasks.
This one is not one of my suggestions, it was made by Mark Reynolds in the user-to-user forum after my new styles suggestions, but I support it wholeheartedly:
Noise - at the moment its possible to use Pattern Overlay and generate patterns to put grain texture and noise into layers. But a layer style with the correct parameters, grain size, blur, intensity, saturation (Saturation applies to the saturation of the noise not the image), would be useful.
This brings me back to feature request made about a year or so ago, that it woud be possible to sample overall image 'noise' or 'grain' and to apply this to areas. It would use image analysis techniques, although Im not sure yet what these would be - to create an 'average' grain structure for the existing image (probably the base layer but this could be redefined). This would mean that any bad retouching (blurring or image softening) could be more easily salvaged, and would also mean that its easy to do much more destructive editing, more freely on an image than before, without the worry of banding on output.
I hope that other users will chime in with proposed refinements, or additional (useful and feasible) layer styles they'd like to see.