The Fantasy Trip/In the Labyrinth showed me a system’s complexity is not intrinsically linked tho the enjoyability of the game.
From RIFTS, I learned not to be too worried about “balance” as long as everyone is having fun. Never was in a game with Vagabonds & Glitterboys in the same party, but people never griped about power disparities beca they were enjoying playing their characters their way.
From Traveller and Central Casting, I discovered character history can be impactful for a character’s present and future.
Champions/HERO showed me the joys of a flexible, point-based system. In particular, there’s usually more than one way to model a given character concept. While that’s particularly true of supers/toolbox systems, it’s applicable to most (not all) systems, and I’ve used it a lot in 3.5Ed D&D.
From MechWarrior, BESM, Prime Directive, Stormbringer/Hawkmoon/Corum and many others, I figured out that sometimes, a system designed from the ground up to support a certain playstyle will do a better job of delivering on that playstyle than simply running it in a syste that wasn’t.
Mutants & Masterminds illustrated that system CAN matter, and quite profoundly. I wanted to run an old campaign I had run in HERO with a new group, but nobody else wanted to play in HERO. So I figured M&M was the next best thing, and on many levels, it was. However, certain dissimilarities to HERO manifested in D20 mechanics…that also differed from 3.5Ed mechanics the players vastly preferred. Those mechanical quirks were a substantial factor in the campaign’s failure.
Playrests showed me no game is flawless; they’re all breakable. And THAT fact gave me a litmus test for my fellow players. I figured out I could game with casual gamers, veterans, and “optimizers”, but the players who routinely seek to exploit the fault lines in systems are no fun to game with beyond a one-shot. They’re not pleasant for me to game with long-term.