I'll add another feature that make hang gliders unique, while I wait for other member's brains to kick in.
Hang gliders have sails ! ! ! Sails are typically found on "Sail Boats". But when an aircraft has a sail for a wing, . . . that's unique!
I've thought many times about how, if someone a thousand years ago had thought to take a mast from a sailboat, multiply it by two and join those two masts together (to form two leading "masts"), and with the boom acting as the keel, . . . a hang glider could have been born way back then.
Cables (or ropes) and stays have long been part of a sailboat, so upper and lower "rigging" (a sailing term BTW) for the thousand years ago hang glider (or name given to it way back then - Sail Wing or Sail Bird) could have had similar mechanical support as compared to today's hang glider.
Launching such a wing would have best used a rail and roller system down a moderate to steep slope. This, considering that both the LEs and keel would have been wooden spars and the sail, canvas. With all this I do think flight could have been achieved (a safe landing ? ? ?). But, I also think that a contemporary hang glider pilot would be the most likely to think of all these possibilities and only IF they could somehow go back in time.
