I am a guy who plays games for fun. Achievements mean very little to me, but I do go for them if I am able to.
If they are near-impossible tasks like "beat the whole game on the hardest difficulty while firing no bullets, killing no enemies, and gathering every piece of intel" then I don't even try for them. Especially when games have it where the bad guys can kill you in one shot while you need to hit them 50 times with the strongest weapon to kill them. You are supposed to be a bad ass and you cannot even beat the first level because the game cheats you out of it.
I understand that challenges are fun, but not when they are damn near-impossible to do or take an over amount of obsession to do.
If I can say, most multiplayer achievements are my worst enemy because I am not good at multiplayer games. I remember FEAR 1 where I beat the game on the hardest difficulty and I only got like 10 achievements out of 47 and only got 50g for all 10. The rest were so unreasonable that I just never played the game anymore. Even on some of the gaming websites they say FEAR 1 had one of the hardest achievement lists to pull off.
I think there are only two games where I got full 100% achievements. Halo 3: ODST and BioShock. (No I do not count the "Play the game on hardest difficulty without dying" as this is near-impossible and was created after the game was released in the last update for the game)
One last thing, I also had this problem with COD4 on the hardest difficulty. It CHEATED me out of the achievement on one level. You have to get into a helicopter in the "All Ghillied up" while carrying someone slow as hell with guys who have 110% accuracy, x-ray vision, and so on. I got ONTO the plane and by this time, the game doesn't let you do anything. You are stuck in the plane. I figured "whew, I made it after dying 50,000 times," but no this was not good enough for it. An enemy throws a grenade right near the plane and it kills me. I could avoid it, I couldn't even move. I could sit there and slowly watch myself get blown up.
It cheated me out of the achievement and that was when I gave up. You think the game would have given me invulnerability when you got on the plane, but no it doesn't.