Some of these are his personal preferences for the type of game that he wants to make, and some of them are his general opinion of what constitutes good game design
No scoops, use up-posts instead (source: I forget where he said this)
No two flipper games, must be three flippers or more (source: https://www.thisweekinpinball.com/this-week-in-pinball-july-8th-2019/ )
He said that his games will have lots of flow and lots of state-change*. I always heard people say that the two design philosophies of pinball are flow vs. stop/start, so hearing his different pinball dichotomy was pretty fascinating. And of course, the new way of thinking of things isn't a spectrum, it's a graph with an x-axis and y-axis. (source: same as above)
No repeatable backhandable ramps (source: https://www.thepinballnetwork.net/e/final-round-pinball-podcast-ep-40-overboard/ )
Other things I've noticed:
3+ ramps on every game (counting the jump ramp on Iron Maiden: Legacy of the Beast, and the Captain Marvel ramp on the Premium version of AIQ)
Doesn't feel the need to include interactive toys on the pro model, which no one minds given that his games have highly praised geometry.
Not putting pop bumpers in the boring old "lanes above pop-bumpers" pattern.
Both Iron Maiden and AIQ have state change in the form of something that raises up out of the playfield to give access to a subway.
All three games have captive balls in one form or another.
*A definition of State-Change: When the pinball machine has a toy which causes the shots available to the player to change in some way. For example: the castle in Medieval Madness has three states. Drawbridge up, drawbridge down, gate up. Other examples: the revolving bookcase on Addams Family, Frankenstein on Monster Bash, the Balrog on Lord of the Rings, and the disappearing pop-bumper and the ringmaster on Cirqus Voltaire. I would imagine that diverters that direct a ball to a different place when a shot is made would also count, at least arguably.