Highly detailed mechanics with tons of options, as noted above it bogs play down.
More importantly for me though is that unintended consequences/synergies leave you with less realistic results than simplified rules. Also the highly complex systems take away from the cinematic feel of the game. I grab the chandelier and swing across the room should be an exciting event, not 15 different die rolls on 8 different tables.

As a player: Knowing everything about the rules, setting and potential enemies as possible. Reading the monster manual from cover to cover, knowing all the mechanics for all the actors on the table, building a character with optimization for the next 10 levels in mind at character creation. Now it’s much more fun have a sense of discovery and excitement about new things. If it means taking the suboptimal approach most of the time I’m much more cool with that than grinding for the optimal outcome against things my character has never encountered before. It does mean playing a more forgiving system though as some games out there are built for the Hardcore Henrys