I have a very high threshold for "boring" gameplay. What gets my goat are straight-up bad mechanics or base imbalances.
Dark Cloud 2: I was playing just fine and enjoying the whole game, but got depressed when I found out the monster tranformation mechanic was underpowered garbage. Ruined the whole thing for me after a while since I wanted to kind of 100% the game.
I quit World of Warcraft a long time ago for a variety of reasons, but mostly because the PvP was junk. Despite all the classes being as balanced as stilts on a treadmill, there are a few specific things I hated. Graveyard zerging in the open world was an awful mechanic. Yeah, everyone hated having their corpse camped, but I'm talking about when you kill some suckers but they just don't stay dead. Their bodies are going to randomly rise and you can never get rid of them. If you wanted to be safe, you had to kill them and just leave the area. Hit and run was sometimes fun, but large engagements easily became huge cluster Fux. Other MMOs fixed this by making you simply respawn in a faction graveyard when you died.
Burning Crusade introduced flying mounts which made open-world PvP worse. There was no continuity to the battlefield. It was easy to avoid other players, zip around everywhere without consequence, and randomly inexplicably get ambushed by death from above you probably couldn't have seen coming since you were too busy killing things on the ground.
The biggest PvP factor for my quitting was that arenas were the only endgame PvP at the time in Burning Crusade. Having the reason you play the game amounting to a boxed-in arena with some pillars and ramps to mix it up wherein you deathmatch again and again and again against min/maxed broken bullshit class combos got boring and depressing. After I gladiator ranked season 1 of the arena, I was already through with it.
The absolute worst insult was when they ruined Alterac Valley. This big epic PvP battle zone turned into a PvE racetrack to kill the end NPC boss. Complete terrible garbage. You would actually get yelled at by the other team for being turtles if you ever tried to engage them directly in PvP combat... in a PvP battleground.
Guild Wars 2 I quit because the innovative MMO combat system thing just didn't work well enough to be really fun on its own. The two dodge rolls were the only truly unique thing about it, but they tried their hardest to milk that for all it was worth. No class roles meant that all the PvE was like a group of all DPS classes in another MMO trying to play a dungeon. Lots of kiting and trying to burn everything to the ground as fast as possible before problems could arise, which is really just silly. As a result, all the PvE content had to be really easy to facilitate this playstyle philosophy to the point that that you could almost solo most of it with the right class and build, which people often did. You didn't need to really think about your class role in relation to anyone else. It always involved 3 simple elements; Spam abilities, kite, and mash your heal cooldown. This was the same for every class. It made me want to play a traditional MMO often.
Crafting was also pointless. People only did it to say they did it and to craft specific grindy things for legendary weapons. You could buy everything right off the broker for less than the cost and hassle it took to make it.
The big World Vs World PvP mechanic was a very good idea, but had some draggy execution all-around. Poor matchmaking meant one world would often get stomped and just lose all morale for the rest of the match week. The worst part is it was often just whoever had the biggest zerg was the winners. If you had a small group vs their zerg you were probably screwed. Sure, some guild could rally people together and actually pull off tactics and shizz sometimes to turn things around spectacularly, but all the hours spent "Player VS Dooring" or being hopelessly outnumbered or just getting raided at night by different timezones didn't make those moments seem worth striving towards in my opinion. What it needed was incentives for small groups. There was no reason not to just bunch up into the biggest group possible and zerg individual objectives.
Also the "no enemy players in the PvE gameworld" and "everybody is your friend in sharing, questing, and life all the time without exception" design about the whole game just reeked of carebear communism. I was able to stomach it for quite a while, but it wasn't for me.
Honestly, there aren't too many things that make me quit a game I've decided that I wanted to play. Like I've said, I can handle a fair amount of boring, difficult, repetitive things. It's really when I get distracted by another game that I have to quit games I like.
TL;DR Dark Cloud 2 monster tranform mechanic sucked. Guild Wars 2 was meh. Everyone knows WoW sucked/sucks. I tend to like most other games that I decide I want to play.
Dark Cloud 2: I was playing just fine and enjoying the whole game, but got depressed when I found out the monster tranformation mechanic was underpowered garbage. Ruined the whole thing for me after a while since I wanted to kind of 100% the game.
I quit World of Warcraft a long time ago for a variety of reasons, but mostly because the PvP was junk. Despite all the classes being as balanced as stilts on a treadmill, there are a few specific things I hated. Graveyard zerging in the open world was an awful mechanic. Yeah, everyone hated having their corpse camped, but I'm talking about when you kill some suckers but they just don't stay dead. Their bodies are going to randomly rise and you can never get rid of them. If you wanted to be safe, you had to kill them and just leave the area. Hit and run was sometimes fun, but large engagements easily became huge cluster Fux. Other MMOs fixed this by making you simply respawn in a faction graveyard when you died.
Burning Crusade introduced flying mounts which made open-world PvP worse. There was no continuity to the battlefield. It was easy to avoid other players, zip around everywhere without consequence, and randomly inexplicably get ambushed by death from above you probably couldn't have seen coming since you were too busy killing things on the ground.
The biggest PvP factor for my quitting was that arenas were the only endgame PvP at the time in Burning Crusade. Having the reason you play the game amounting to a boxed-in arena with some pillars and ramps to mix it up wherein you deathmatch again and again and again against min/maxed broken bullshit class combos got boring and depressing. After I gladiator ranked season 1 of the arena, I was already through with it.
The absolute worst insult was when they ruined Alterac Valley. This big epic PvP battle zone turned into a PvE racetrack to kill the end NPC boss. Complete terrible garbage. You would actually get yelled at by the other team for being turtles if you ever tried to engage them directly in PvP combat... in a PvP battleground.
Guild Wars 2 I quit because the innovative MMO combat system thing just didn't work well enough to be really fun on its own. The two dodge rolls were the only truly unique thing about it, but they tried their hardest to milk that for all it was worth. No class roles meant that all the PvE was like a group of all DPS classes in another MMO trying to play a dungeon. Lots of kiting and trying to burn everything to the ground as fast as possible before problems could arise, which is really just silly. As a result, all the PvE content had to be really easy to facilitate this playstyle philosophy to the point that that you could almost solo most of it with the right class and build, which people often did. You didn't need to really think about your class role in relation to anyone else. It always involved 3 simple elements; Spam abilities, kite, and mash your heal cooldown. This was the same for every class. It made me want to play a traditional MMO often.
Crafting was also pointless. People only did it to say they did it and to craft specific grindy things for legendary weapons. You could buy everything right off the broker for less than the cost and hassle it took to make it.
The big World Vs World PvP mechanic was a very good idea, but had some draggy execution all-around. Poor matchmaking meant one world would often get stomped and just lose all morale for the rest of the match week. The worst part is it was often just whoever had the biggest zerg was the winners. If you had a small group vs their zerg you were probably screwed. Sure, some guild could rally people together and actually pull off tactics and shizz sometimes to turn things around spectacularly, but all the hours spent "Player VS Dooring" or being hopelessly outnumbered or just getting raided at night by different timezones didn't make those moments seem worth striving towards in my opinion. What it needed was incentives for small groups. There was no reason not to just bunch up into the biggest group possible and zerg individual objectives.
Also the "no enemy players in the PvE gameworld" and "everybody is your friend in sharing, questing, and life all the time without exception" design about the whole game just reeked of carebear communism. I was able to stomach it for quite a while, but it wasn't for me.
Honestly, there aren't too many things that make me quit a game I've decided that I wanted to play. Like I've said, I can handle a fair amount of boring, difficult, repetitive things. It's really when I get distracted by another game that I have to quit games I like.
TL;DR Dark Cloud 2 monster tranform mechanic sucked. Guild Wars 2 was meh. Everyone knows WoW sucked/sucks. I tend to like most other games that I decide I want to play.