I'm a 100%-er, and I think being a 100%-er makes me incapable of understanding how or why people play games and not fully complete them.
When it comes to games like Pokemon or Metroid, where the 100% completion is not that challenging, just time taking, then yes, I'll complete every little thing. The only Pokemon game I have 100% on, is Silver so far. I caught every single fucking pokemon, and even got myself a shiny Celebi while I was at it!
Games like WoW or Assassin's Creed though, that have extremely time wasting and obscure 100% completion requirements are lost on me though.
I'll spend alot of time playing the games I enjoy in order to get 100% completion on it, like using guns and equipment that I don't always use in MW2, because its fun. But getting every single challenge done, when some of them are completely ridiculous? That to me, is a complete waste of time.
Throw an egg into the air and shoot it in the air while blind-folded: fun.
Throw an egg into the air while on the back of a moving horse, which is on a moving train, while blind-folded, upside down, and spinning in circles, with the most inaccurate gun in the world: I'll pass.
Even IF it is worth 25 dickhat points.