This is kind of a minor thing, but something that’s been a growing irritation for a while. Photoshop’s Stroke layer effect actually eats into the layer it’s applied to, resulting in an undesired destructive effect if you start doing things like adjusting the layer blending mode or opacity of the stroke.
So, in this image the left window has a black square sitting on its own layer, in the middle I’ve applied a 3px red stroke on the inside of the layer, and on the right I’ve reduced the opacity to 20%. Instead of the red stroke blending into the black square — since it’s sitting on the inside, remember — the white background behind the square pokes through, which means the stroke is effectively knocking out any pixels of the original square.
Not a permanent loss since you can always discard the layer effect, but it seems counter-intuitive to me. I suppose there are situations where this might be the desired and expected behaviour, but more often than not I’d prefer it if the stroke were simply to fade into the layer.
The simple work-around: Cmd + click (or Ctrl + click on the PC) the object’s layer in the layer palette, create a new layer immediately above it, and then use Edit -> Stroke to apply a stroke to this layer. This selects the object’s contour, and applies the stroke pretty much the same way as the stroke effect, just on its own layer.
I suppose this would be less of a pain if Photoshop’s layer management weren’t such a hassle already.