One thing I hate in video games is the "instant death", where with one or two hits or shots or whatever, the computer can completely negate the past half hour or so of game time and end your game.
The biggest offenders I've noticed of this happen to be japanese games. I don't know if that's an element of the culture, or if it's just a coincidence or my dumb luck. With such a small sample size, it's hard to say.
Anyway, the games on my mind in particular are the Dead or Alive series, and Samurai Warriors (especially on the harder levels). Both of these games have this feature where the AI can "juggle" you, where each hit knocks you up in the air where you have no control, cannot block or counter, and it can hit you over and over and over again. Effectively, all you can do is sit and watch as your life goes from full to half or often below.
Not only is it extremely unfair that one series of unblockable hits takes off most of your life, but it's something that just seems impossible for me to do as a player. Dead or Alive seems to be especially cheap in this regard, in that I've seen the computer actually break out of being flung in the air. Lack of skill on my part? Lack of knowledge maybe; if there's a way to do it, it's not obvious to me.
There's also the blocking disparity. It seems the computer is capable of blocking any move at any time, while it laughs at my blocks. How is it that the computer can be standing still, I come at it with any attack (standard, power, charge, whatever is applicable for the game), and it blocks it perfectly; yet when I'm standing in a blocking stance and it comes with a normal attack, it scores a hit? Is it any wonder I haven't really learned how to use the block under stress, when even under the simplest of circumstances it just doesn't work? And yet, the computer reserves the right to block out of any attack or combo at any time, whether they're in the middle of being pummeled by a combo, stunned, in the air, or on the ground.
Sometimes, it's not the juggling, but the enemy will just be able to do so much more damage per hit. When I can sit there and wail on someone (actually hitting, when I get past their block) for a 50-hit combo and they only have maybe an eighth of their life gone, and in two hits I'm near death, that's just not fun. It's frustrating. I encountered that in Project Sylpheed as well, in some of the dogfights against enemy "bosses" — I'll score a few dozen hits and maybe take half their shields, and in half a second they somehow manage to remove my shields and half my armor in a single volley.
Plus, I hate wasting time. I don't have a lot of time to play as it is, so when I spend 40 minutes attacking and parrying and trying to find a way to get health back, only to very suddenly and immediately lose to a two-hit, unblockable, unbreakable attack (or a fifteen-hit juggle, hoping my feet will touch the ground long enough for it to acknowledge a button press from me before scoring the computer's next hit)... I want to throw my controller through the TV, snap the disc into a hundred pieces, and never play again.
If I wasn't such a "completionist", I'd take my 950 points from Samurai Warriors and move on. But I'm so dang close to those last two — yet I've been there for three days, and the game seems determined to not let me have them.