I've never quit a game based off of a boss, but I've come close: Cynthia from Pokemon: Diamond and Pearl - mainly because of how out-of-nowhere her Amazingly Awesome team came from, and how they were such a stupidly high level compared to everything else you'd been fighting at the time. Because of the poor type distribution of the 4th Gen dex (pre-plat at least) all the other elite four were super easy to get through save the psychic guy, so I was able to hit Cynthia with a team of mid-50s - proceeded to get steamrolled something like 30 times because I didn't bring anything along that could take down a 68 Garchomp with perfect IVs, EVs and nature.
I managed to get her down eventually playing the 'Do I have more revives than you have PP' game and by getting so many levels steam-rolling the Elite Four just to get up to her, but fuck me I wouldn't be surprised if people just gave up fighting her. I know the Champion is supposed to be tough, but the difference in power between her and the Elite Four (and the entire game, even) was staggering. Much prefer her in Platinum, where her levels aren't as insanely high compared to when you finish the story so you're not stuck grinding for 6 hours just to have a chance against her.
SlumlordThanatos said:
After three months and 300+ pulls for Heroic Spine of Deathwing way back in WoW: Cataclysm, my guild fell apart. I also took my first "break" after that happened; I was sick of banging my head against that boss with a different healing partner every week.
After that, my tolerance for endgame raiding was never the same. When I came back, my guild stonewalled on Blackhand in WoD, and I just quit playing after about a month of wiping to this boss. Haven't been back since.
Something similar happened to my guild during Cataclysm too - didn't help that after Spine came Madness of Deathwing, which was just... god, so boring, Yet also insanely challenging, since if anyone died it was basically a wipe since you NEEDED so much dps and healing for that last 20%. Plus given it took 10-ish minutes just to get to that last phase... yeah, Dragon Soul can get fucked.
Personally I didn't end up taking a break until Siege of Orgrimmar in MoP and Hellfire Citadel in WoD - both of them really suffered from 'too god damn long' syndrome, which really wasn't helped by how there wasn't anything else to do besides them for over a year respectively. While some of the bosses had fairly interesting mechanics, the fact they took so long to get to and you had to fight through all the shitty 'murder all the add' fights just dragged things out far too long for my taste. Add in how you couldn't skip bosses reliably during SoO so you were forced to fight through the half-dozen boring-ass bosses so you could get to the interesting ones... yeah.
Worst part is after I took my break Blizzard made a big song and dance of how people who'd been playing for 10 years straight would get some awesome statue-thing - literally missed out on it by a couple months because I couldn't be bothered fighting Galakras for the hundredth time. Ah well.