I've been experimenting.
Postboxes? no problem.
Traffic lights? got it.

Ran into trouble with a power substation though.
I created a closed way and tagged it to make an area that is a power substation.
Apparently I'm missing something because while other changes I've made have all been rendered, this still is only visible in edit mode:
osm.org/?lat=40.74106&lon=-74.041854&zoom=18&layers=B000FTF

What did I do wrong?

Also, I had little luck marking a taxi stand as a node on a way. The data is still there, but it doesn't show on the map :-( (maybe taxi stands just don't get rendered?)