Since you focus so much on user-friendliness, I imagine the typical Mapzen user will tend to base their mapping and tagging practices on what looks good in the Mapzen editor versus the correct way to map and tag features. Therefore you must strive to make sure that the correct way looks best in your editor. You can start by making sure that if a polygon has (for example) the tag “amenity-college” then an appropriate icon should be placed in that polygon, just as if the tag were on a single node.

P.S. Potlatch also fails to draw POI icons for polygon-drawn features, but I think its users pay more attention to correct tagging because Potlatch doesn’t try to draw everything in an “intuitive” way to the extent that Mapzen does.