I would say the code being deeper on some modern Stern is what holds my interest. Rulesets on Stern games such as those found on LOTR, XMEN, Simpsons, and Star Trek are deep and multilayered. However, I find the rulesets on some other modern Sterns to be rather boring such as those on ACDC, Metallica and POTC which tend to be very bash heavy and require hitting the same shot over and over again (ex, hit the left ramp 15 times to start multiball, bash this target 50 times to get an extra ball, etc).
For me I prefer games that work an entire playfield in interesting ways with a quest based ruleset to be the most engaging. There are Bally Williams games that pull this off better then modern Stern's. In my opinion the rulesets for Star Trek: The Next Generation and The Shadow are more interesting then say ACDC's as I prefer the rule design in those games.