There is the nicely subversive implication that those who are wealthy and respectable today are just the heirs of the criminals of old. Generally this is inferior both to its model in Feuillade (even the black masks and body suits) and the later Lang such as Dr Mabuse and Spies. To dream of what might have been done with the character of the diamond king's daughter just look at the comparable figure in Judex.
The cinematography, composition, and interesting characters (especially the female villain) are what make this more than just an average adventure film. It's not surprising that this was popular at the time, because there certainly are a lot of elements for a box office hit, but within Lang's body of work this probably doesn't yet show his entire genius.
Sprawling adventure epic involves spies, lost treasures, and angry natives as a wannabe adventurer finds a mysterious message in a bottle, setting him on a worldwide search for a priceless diamond. Hot in pursuit is the deadly spy organization, The Spiders, who will stop at nothing to get their hands on the diamond. A predecessor to Indiana Jones, Lang directs with a flair for action and mystery.