A Long Yellow Request
This sign impressed me based on numerous qualities:
1.) the level of explanation; it's clear that the author(/s) couldn't imagine issuing an order without explaining the rationale behind the order,
2.) the evenness; the font size, the layout - everything seems to suggest that this sign comes from someone who values equality and possibly is directed to an audience with similar values.
3.) the color; the author went out of the way to provide a color that would "jump" a little off of the white door frame where the sign is posted, but not in a way that's too excessive of self-important.
4.) and of course, the polite conventions that often frame such signs: a "Please" at the outset and a "Thank You" at the end.