The tag wiki for the plot-device tag says:
An element introduced into a story, film, play etc. to advance the plot.
This is really not a good definition of a plot device. A plot device is a narrative technique used to advance the plot. Not every element of a story is a "plot device" even though almost every element of a story probably affects the plot in some way.
A plot device is really a "key" element of a story that drives the plot -- without which the plot wouldn't advance.
For example, in Lord of the Rings, the One Ring was a plot device, but Gimli's axe isn't.
Looking at the questions tagged plot-device a good number of them seem to be using it in the same way the previously-burninated plot-explanation tag was used, i.e. "I didn't understand this part of the plot, what happened?". I think the overly broad tag guidance may be playing a role in this.
As it stands, with the current guidance, nearly every non-story-id question on this site could be tagged plot-device. Should we try to make this better?