I see it as a cooperative effort by all sides of the table. Both the players and the GM should be having fun.
If the GM is getting sadisitic all the time and dropping Force 30 spirits with all the great dragons and 20 different battalions of Red Samurai on the players every single session, players aren't going to have fun.
At the same time, if the players ae running around with mil spec gear, have full acces to Thor shots, and whose solution to every mission is to use those Thor shots, or decide that they want to kill all the great dragons in the Sixth World, and are able to one-shot kill every single enemy and it's impossible to hit them because they min maxed everything about their character...chances are it's not going to be fun.
For me, no going nuts with super extreme Dark Souls-esque campaigns, and not letting your players pull off batshit insane Saint's Row IV campaigns either.