From my quick read of tao, it seems to be doing essentially what graph databases do but with the data storage layer being in sql rather than some other object store. And with the interface layer being not quite as feature complete. So the query layer in tao seems to lack a way to follow multiple edges without first returning to the application code, which graph dbs present as a native feature. The other thing that's lacking, which some graph dbs provide, is labeled edges - that is edges that contain data besides just an "edge type".