4E. More specifically, hitting 10th level in 4E and realizing that the experience was pretty much the same as 1st level, just that combats took 10x as long. The cool, high-level enemies we thought would be exciting to battle (demons, rotting undead necrotic-bile-spewing dragons, etc) just turned out to be drudgery and grind - most sessions towards the end we got little if any exploration or RP in, because once we hit an encounter that would suck up the rest of the evening (and this was well after we got on board with having power cards prepared for each player).
I tend to play with the same GM I've known for the last 15 years so most of my 'screw you guys, I'm out' stories aren't quite as acrimonious as some here, but that was the first time when we had an all-round player revolt (well, 3 out of 4).
Sucks, cuz I loved the character I was playing... one of a very few paladins of the R*ven Queen (death goddess), guided by spooky visions to combat a growing plague of undead spilling into the world. LG and all like any paladin, but he was... unsettling as all get out to most people, including the rest of the party. Pale as a sheet, humorless as a tax accountant, driven as a nail, talked in total monotone with unnerving frequency about all the time he'd spent up to his eyeballs in corpses, rode a half-celestial nightmare (not sure I want to explore the backstory on that particular conception), and generally was as feared by the people he was protecting as he was by his enemies (til the poop hit the fan, at which point his utter unflappability became an asset). Probably the only paladin I ever had undiluted fun playing and for whom absolute, obsessive LG-ness made sense (you gotta have some kind of code to keep you sane when your daily activities include wading through a chest-high stew of rotting corpse funk and cutting your way out of the belly of a zombie behemoth with a severe case of indigestion).
But the dissolution of that campaign brought me to Pathfinder, so it's all good. Our GM was a little miffed that we all conspired to up and quit rather than approaching him individually, but it worked out in the end. Great GM, but funnily enough he can be kind of a dick player - he was once miffed enough to quit cause his GM in an Amber DRPG game wouldn't let him be from a shadow with dinosaurs where the Jurassic Park theme song played on a continuous loop in the background (everywhere, forever) and got ejected from a Dark Heresy campaign for doodling 'a dragon with D6 penises shooting out of its mouth' on the battle map to represent their enemy while the other GM was trying to set up for an encounter.
