You'd be pretty hard-pressed to find me playing on high difficulty. Mostly it's because of bullshit penalties that the game inflicts on you that isn't present on higher difficulty settings. A perfect example is the Left 4 Dead games. Left 4 Dead and L4D2 penalize people for friendly fire on higher difficulty by allowing you to damage your allies (a common griefer thing), and I utterly suck ass trying not to get hit with friendly fire penalties. I'm serious, in any given Left 4 Dead 1 campaign I'd rack up anywhere from 14 to 25 friendly fire penalties and in Left 4 Dead 2 is no better, hitting somewhere between, well, 14 to 25. Part of it is that I aggresivley try to assist by killing zombies munching on my allies' asses and part of it could be my weapon choice. I usually take the Deagle and either the Combat Rifle, Auto-Shotgun, Combat Shotgun or Sniper Rifle, and this can prove bad because the Deagle is a piercer (its bullets will go through multiple zombies and often this leads to me hitting allies), the shotguns are of course shotguns and have a wide spread, the Combat Rifle's burst mode hits zombies fine but that third shot ends up hitting allies as well, and the Sniper Rifle I just suck with. It also doesn't help that allies are ALWAYS jumping in front of me, so maybe those jerks deserve it for because heaven forbid they can shoot a zombie by standing THREE INCHES TO THE RIGHT!! (Yes, that was a SpoonyOne SWAT 4 Let's Play joke.)
Then there's the achievements. Look, if you can get the same achievements on a lower difficulty, what's the point of playing on a higher difficulty? I mean other than having to deal with more unwilling-to-die cookie-cutter baddies and fiddling with annoying controls and the bad camera that seems to be in most games.