Quoted from zacaj:
I think the first turning point was after Black Knight when williams system 7 came into play. Code started getting way more complex and probably outpaced what players could handle at the time. Seems like they actually simplified a bit in the system 9-11 era. Then around TAF it increased again, then finally in the mid-late 200s
I'll agree with Zac here from the programmer end of things, Stern mpu200 and WMS system 7 is where the emphasis had shifted from "get the thing running" to "look at the cool stuff we can do here, easier". Which did translate to a layer of depth but some of which was lost on the player - most of the ease of coding involved background stuff like light shows which are hard to code correctly (thanks to most of these operating systems' multi-threading).
Player end depth I think it kept progressing through each era, culminating in Keith's Stern/JJP games. Ridiculous depth there.
What's actually REALLY hard to to have *accessible* layers of depth, where there's easy stuff for player to do if they're not into crazy depth, but also something there for advanced players to do as well. AFM would be the gold standard here, followed by most of Lyman's games. If you play skillfully, you can build up to something really special. (ac/dc comes to mind).