Question says it all. You have an achievement, power-up, or equipment/weapon that basically says "I've went through the fiery pits of Hell and back for this!".
I recently done a no-alert playthrough of Metal Gear Solid 4 to finally get that stealth camo. Did many do-overs an insane amount of times. I also did a no-kill playthrough for the bandana. That was also a *****. Also, the Marathon-Pro perk. I have to do this over and over again because I just hate playing Capture the Flag on any game.
It could be anything, really, as long you think you worked your fingers to the bone for it! Doesn't have to be lengthy.