S.T.A.L.K.E.R.: Shadow of Chernobyl... I was having a tremendous experience with the game. Really. It was atmospheric, immersive, challenging (autosave is bound to my M3), and really kept me wanting to make progress and see everything it had to offer. Then it stopped being fun. Why? Two words: Lab X16. When I showed up at the lake region, I had two severely awesome assault rifles, a pistol, a fuckton of grenades, and enough bullets to fill up a pillowcase and beat someone to death in case my guns broke. These treasures were all very hard-earned from several days of exploration and cautious firefights.
Then suddenly, SNORKS AND ZOMBIES EVERYWHERE. By the time I got to the freaking lab, I had about one magazine left in my upgraded/scoped AK and a couple of bullets for my Walther-facsimile. Once inside, suddenly, SNORKS AND ZOMBIES. EVERYWHERE. AT CLOSE RANGE. AND VIRTUALLY NOTHING TO FUCKING KILL THEM WITH. In most games, this would still be do-able by keeping a cool head and aiming true, but this is STALKER, where bullets consider your aim to be more of a suggestion than anything, and for some reason, the enemies that it's fond of throwing at you in huge numbers are even harder to kill than the ones it sends after you sparingly (at one point, two snorks ate at least three of my most powerful grenades before I had to frantically finish them off with a piss-weak handgun), and as the cherry on the "fuck you" sundae, on the rare instances that they do drop ammo, you'll get back a third of what you just put in their skulls. By the time I got to the part with the Brain Scorcher, for the first time in my life, I found myself playing a game where I had no weapons and the zombies were shooting at me.
As you probably guessed from the subject of this thread, I said "fuck this", exited out, booted up the much-less-aggravating TF2, and have yet to give it another chance.