Artificial lengthens, it's really tedious to do the same thing 7 times, with a slightly different goal (this is mainly in rpgs, but could be in other genres)

Well losing your data is always a killjoy. Whenever I lose my data, I usually just giver up on the game. Another one would be a really hard level, like in Donkey Kong Jungle Beat. I cannot remember which level it was, but I could never beat it, so I stopped playing the game.

I experienced multiple killjoys in the original Borderlands. I lost two save files to corruption (gah, power surges). The worst part about it was that I had already managed to get through a decent chunk of the game before losing the characters. My first character was a level 27 Roland and the second was a level 32 Brick. Another might be the Borderlands hacks that plague the multiplayer. When I joined a hacked lobby, it completely ruined the story-line for my level 47 Brick, and it forced me to either level up in a really asinine, tedious way, or stay in the hacked lobbies, both of which caused the amount of time I had to play to get the platinum to increase dramatically. The only other killjoys I can say I have experienced would be playing a multiplayer game and getting placed into lobbies with highly experienced players where you would lose a ton of experience if you left (or lose a ton of experience if you stayed), and also the random glitches where you fall through the game world, and you end up dying and losing progress.

My old Kingdom Hearts 2 profile got corrupted. I spent around 110 hours on it and I just barely got Final Form when it corrupted.

The insta-deaths that are available on some games where if you don't respond fast enough, you die.  Resident Evil 4 is a great example, where you have to run from a boulder, free yourself from a snagged rope, etc. and if you don't button mash quick enough or make the decision fast enough, you die.

It isn't skill, it isn't an ambush you can react to, it's just so fast and so irritating it makes me unhappy.  Those kinds of games aren't the greatest and the numbing effect the instakills makes all your achievements seem like nothing.  You want to save all the time because you'd have to repeat the action over and over until you got it right.

No thank you.

The biggest game killjoy I've experienced is playing through an hour of Skyrim without saving, having the game freeze on a loading screen, and having to do the same dungeon all over again. So disappointing!

In-game killjoys:
-Forced repetitions of a single task is always a pain and lowers the enjoyment I get from a game.

-Cheapshot "difficulty". You know what I'm talking about, it's those times when a game drops you smack dab in the middle of a level or challenge and either leaves you with no idea what you're supposed to be doing, and you die fifty times before you figure out what's even going on. Or worse, when you're expected to know the layout of the level or the placement of the enemies, even though it's your first time playing the level. Give me a real challenge, not a cheapshot. Something that challenges my skill.

-Suudden and extreme difficulty spikes. Those moments where the game goes from being a breeze to kicking your butt in the space of ten seconds. Level eight should not be ten times more difficult than level seven.

-Insane bosses. A perfect example is Shadow Dancer. I once got through to the final boss with around thirty lives, plus three or four three life continues, and lost every one of them against that boss. It actually requires you to fight off a few waves of ninjas, stand in the exact right spot when the boss attacks, then attack him and hit his weak spot during a very brief window of opportunity every time you attack him. Succeed four or five times in a row and you win. It's the most difficult boss I've faced.

My last killjoy experience was cheapshot difficulty in Splinter Cell on the GBA. It was so bad that I lost interest in playing it again. It's a shame too, because it was a good game.