> the points on a map have to represent one of a fairly limited selection of types Absolutely not. Not only is the selection of types huge and full of shades o

> the points on a map have to represent one of a fairly limited selection of types Absolutely not. Not only is the selection of types huge and full of shades of grey, but an element can be tagged as things from multiple different categories at once (and this is a wonderful feature). Take a browse of the OpenStreetMap wiki sometime. OpenStreetMap data is far richer than a layer model could cope with. However, let's not confuse this with an editor having a layers-like interface feature to help the user focus on a particular subset of the data. JOSM for instance does this very successfully with the "filters" tool, which I use heavily.