Hinges!

Hinges on a spectacle frame can tell you a lot about the quality of the frame itself. I know? Would you believe it? It can help you distinguish between what I would call a mass market/ mass produced frame where they (the manufacturers) want as many people to buy them, so they make them to fit as many people as possible - hence the spring hinge - and a better quality hand made frame, usually made by a more independent frame manufacturer.
The main difference is that the spring hinge is the weakest point on the frame and it will snap at some point.