osm data attributes are incomplete after loading into Postgres database

It is completely normal that for most objects, most columns are null. For example, there might be columns for name, opening hours, reference number, oneway, max speed and so on - how else would they be filled for a post box, or a tree? If you dislike the many "null" columns, have a look at the "hstore" facility that osm2pgsql offers, where it is possible to import all values into a single column; you might want to get rid of some column definitions in default.style then.

Nothing wrong there. null is used to signify that "this object does not have this tag", e.g. "There is no name tag for this road in the OSM database". Since osm2pgsql stores all points in one table, and all lines in another table, and all polygons in another table, it has to merge all the OSM objects together. Roads are stored next to footpaths. Building outlines are stored alongside lakes. Buildings will have a building tag (and hence have a non-null value for that column), whereas lakes will not have a building tag, and have null for that column.