It's an overall look of thickness, whole body.
Just one part? I'd say traps, the average trainer barely does shrugs, usually with 75# DBs, so most anyone that has built traps has put in effort to lift heavy shit.
^ this
a lot of guys have good chest, arms etc, in fact you can see a lot of guys with those bodyparts and thats all they have.
you see someone with impressive traps chances are they have a pretty complete physique, and he looks fucking powerful.
i agree with what fortress said too- want to see how powerful someone is look at them from behind, not from the front.
nothing looks more impressive on someone than thick traps, cause chances are they have the back to match.