Tip #1: During DXF/DWG import select the layer option "Join" for each layer that requires boundaries to be joined. This will usually take care of 85% of the open/bad boundaries. I rt-clicked to activate join for all layers in the column.
Tip #2: Use Join Interactive to take care of the tricky open boundaries. Basically you just select one object on the boundary and Join Interactive will try to close it for you; if it gets stuck, it will ask you to select the next object on the boundary.
Tip #3: I used De-Embed to automatically determine what should be dark/clear.