I've also found that in 6.4 SP2 Classic UI, the "Drag components here..." area is missing. This is due to the "null" value in the "actions" array rendered by the WCMComponentFilter. Executing the same code without the "null" value renders the drag'n'drop area.
This is NOT reproducible in 6.2 without SP2 installed.
After investigating this issue a little more, I managed to track it down to "/libs/wcm/foundation/components/parsys/newpar/cq:editConfig". This node contains an unexpected "cq:actions" value of "_clear", which the ComponentEditConfigImpl class doesn't know how to render.