There are a number of different ways to load data with osm2pgsql. We should document the options that the stylesheet expects
I would suggest this
osm2pgsql --flat-nodes <location for a 16GB+ file> -d gis -p planet_osm --slim --hstore-all -C <cache memory> --number-processes <number of cores>
This will allow us to move seldom-used columns to hstore over time without breaking renderers
It's documented at the moment. If we change what's needed, we can update the README at that point.