For me one of the most important fun-factors in RPGs are the rules systems.
Different rules = different flavors of even the very same setting, the very same game world.
A game world is for me not that important, as I can always make an adaptation for a rules set I currently like.
Rules determine what I as a player actually DO during the game (the gameplay). And therefore, I like rules that are a joy to apply, that make me interested in their mechanics and their applications.
That does not mean I prefer rules-heavy systems - which for the most part I actually don't. it does mean, I prefer rules-INTERESTING rules systems. Rules where you can apply some nifty tricks as a player, where you can master the mechanics to make the game really yours.
Rules, for me, have much more importance than settings. There is always another game world around the corner, but GOOD rules sets are rare and far between.