As Amy Jo noted, they're iterative; and interesting to me is that they still work hard after identifying the good gameplay principles. The design details really matter! As Gwertzman says, "We compete in a try-before-you-buy market, and we believe competing successfully there is a fundamentally different kind of design." And fun is an incredibly demanding business to be in-- no one has to use your application for their paycheck, after all.
"Our path of development is extremely prototype-heavy," said Gwertzman. "We'll make half a dozen prototypes, and pick just one of those to be a hit casual game. And once we develop that one, it's a very iterative process. It's a sandbox model. We try different things out, and find out what's fun. Only when we find out that the core mechanic is fun do we worry about the art, content, and all the other little details."
"We really obsess over the core game mechanics. In a game like Bejeweled, hardcore developers look at that and might think it's kind of...it's very easy to kind of dismiss it, but we literally spent weeks on just the right way for the gems to fall when you make a match. In a game like that, it's little details like that. How does it feel? Getting those little details right is what we prioritize. So when we're designing a new game, we'll spend months and months prototyping core mechanics."
Their game engine is available for free, I was interested to see.