What made Williams Pinball good?

I'd like to think there's something to titles on Williams being more memorable than some of the modern pinball, particuarly Stern.  As Gary says, they are making "modern retro pinball", and it couldn't be truer.  They really are building retro pinball with a modern twist.  What do I mean by that?  Most of their titles have everything above the playfield.  There are no hidden doors, no mechs that pop out from the playfield, no subway tunnels.  I know this is done partly for reliability, but I know cost reduction and ease of assembly / disassembly also plays a part.

A lot of things were hidden on Williams.  You had a moving gun on T2, Shaker motor on earthshaker, spinning fan topper and spinning discs on whirlwind, mist multiball on BSD, adams family hand that pops out, twilight zone magnet flippers, funhouse toy that came alive.  Every time a new title came out, you wanted to play those games just to see what hidden features you'd discover, sort of like playing a miniature golf course for the first time (at least ones with tunnels that lead your ball to a random spot on another place on the green).

I'd like to implement some of that magic into my revision 2 layout.  What happens when I shoot the ball up this lane?  A hole just got revealed, where the ball go from here?  I just saw it pop out, but where did it come from?  That ramp looks different, what's the ball going to do when I shoot it up there?