I am active again, lots of other duties in between, but I am back. I finished the buildings and addresses in my home area Wendhausen (http://osm.org/go/0G4Mkdo_) and I started with 3D tagging. Currently still in time of learning and see what goes best, asking community on whats best. But I already learned a lot.
If you start mapping in 3D, have a look at http://wiki.openstreetmap.org/wiki/Simple_3D_Buildings and follow some guidelines:
always user building=* for a building (even for outlines of building:parts)
always use a relation of type=building for buildings containing building:parts
in multipart buildings set height=* for ALL parts except outline polygon which contains the building=yes tag
attach entrance to building:parts polygon, not only to the outline polygon