UPDATE (2014-07-25): He confirmed!!!
Mark Rein·Hagen himself confirmed that this is a "pretty cool summary" of his strategies. Thanks Mark!
Nothing unusual (which is good!)
From The Gentleman Gamer interview with Mark Rein·Hagen and The Gentleman Gamer Interviews Justin Achilli, developer of Vampire: The Masquerade, it is clear that White Wolf have always used a really standard game design cycle:
- Create a prototype
- Play it, and let it fail
- Learn with the prototype failure
- Go back to step 1
Jesse Schell, who worked not only in video/computer games, but also in thematic park games, argues in his amazing book that a game designer in whatever medium should always work in a process like that. I don't know if Mark Rein·Hagen followed this on purpose or by gut feeling. Either way, he followed a process that many huge studios failed to see in the past. Even nowadays, these things aren't regarded as common sense. And Achilli hinted that they still use this process, citing The Art of Game Design as a great reference.
Although this process is regarded as standard by successful game designers, it is by no means complete. Schell argues that it is by using the right tools (the so called lenses in his book title) that the designer creates a great game. He shows and discuss many tools, and each one of them has a use for any kind of game. So if there is something like a "White Wolf Secret", then here it is: it's tools selection.
From my research, I could find some tools that were used in one way or another by White Wolf. Some of these tools may be really hard to use in other kinds of games, simply because they are not as generic as the ones that Schells lists in his book.
Mark Rein·Hagen stated that he spends a lot of time designing the archetypes which the players will assume with their characters. These archetypes are fun to follow and are also fun to contradict -- so the player is not constrained by the archetype. The archetype is a guide, and the players should choose if they will follow it or not.
Without the archetypes, the players have a hard time finding goals in the game. Their characters become detached from the setting and have unclear behaviour, giving the players a hard time. With archetypes, the roleplay becomes easier.
Mark Rein·Hagen also stated that politics is a major feature in his games. It may seem obvious, but what he defines as politics need not be so openly politician as in Vampire: The Masquerade. He defines politics as a three step process:
- Give people something to believe
- Now give them the means to fight for their vision
- Let them figure out the rest for themselves
Finally, the culture is also really important. Mark Rein·Hagen again stated that he is obsessed with myths and that his first inspiration source is the work of researchers such as Joseph Campbell, who wrote The Hero With a Thousand Faces. So he employs old and new jargon, folklore, legends and gossip to create a mythical atmosphere.
White Wolf uses an iterative process that is gaining recognition day by day as THE process. It's simple and even obvious, and can be applied to all game types: computer games, RPGs, board games and even sport games.
The tools that White Wolf chooses to emphasize are what give its games their characteristic flavour, and so it is with any other game studio. BioWare also distinguishes itself from its competition by the tools that they choose to emphasize. As does Wizards of the Coast. And Evil Hat Productions. And so on.
By the videos linked to in this answer, I found out three tools that White Wolf emphasizes: archetypes, politics and culture. This list is by no means exhaustive. There may be many other tools that White Wolf employs, and that I could not find out. Yet, I believe that these three cover most of their works.