Well, you've said better than I. IMHO, PostGIS is geared toward building classical GIS databases, while OSM is more a Topological Catalog. If I'm not mistaken, they say so themselves and visual map are just a by-product of geo-referencing "everything". Way more complex and way more harder to work with.

Edit: maybe I should have said "higher level geometries" than "more complex". In the sense that OSM "knows" about point and edges and nothing else.