"countable" should also use the Quantity value type, but should be restricted to positive integers, and default to +/- 0. This would remove the awkwardness of using the "quantity" data type for things like the number of neutrons in a carbon core, or the number of planets in a solar system. For properties like "population", +/-0 is unlikely to be true, but the current default of +/-1 is hardly better for that.
Migration note: Existing properties could be converted from "quantity" to "count" by updating the Property entity (the API doesn't allow this, but we could do it via a maintenance script), and then re-generating the property-info entries for those properties. That should be sufficient.