Cool subject , shows that we all want something different in some aspects, and same in others. Must be why we argue about games play-ability and acceptance when a new one comes out
In no particular order :

Drop Targets- multiple sets ,quick visual of what needed to complete set task
MUST HAVE Hand drawn art - no photos of anything ( GB & AS is my style)
Dark colored pf, purple , green, blue . especially purple ! ( Chrome ball on dark background,looks kick-ass too) No all yellow pf's please.
Ramps, easy ramps from lower flippers, and a more difficult upper flipper ( ES fault ramp) ( ST warp ramp) I like clear plastic ramps over metal ones. Better visually and lights from under.
Ramps that return to inlanes - love flow and like to see ball cross pf to opposite side
3 or 4 flippers, across pf shots only that flipper can hit ( TZ piano or camera shot),
Ramps, did I mention ramps?
JP