Wow... I actually did break off a friendship last summer. It definitely wasn't something I was accustomed to, so it hit me pretty hard. I'm just recently getting over it all and am still questioning how we ever managed to get along in the first place.
I'll keep this short. Out of what little respect I have for this person, I'll call her "Cirno" (you Touhou fans will understand both the irony and the significance of that handle). Things were great at first, but as time went on, the more we learned about each other the less we had in common. Add to the fact that she was spiteful (which I can't stand), apathetic ("True Neutral" deez nuts), and silly to the point of childish. When she eventually chastised me for asking her a simple question for the where she only needed to tell me two fucking words, I finally put my foot down, told her to go fuck herself, and went the fuck on about my business. I hate that it ended the way it did, but I think us going our separate ways overall worked out for the better.
It wasn't all bad, of course. But the bad definitely outweighed the good.
I'm currently going through some tumultuous times with some friends on Xbox Live. And just so everyone knows, I've known these guys for years; I'm worried, but still preparing for the worst all the same. I got some words in mind should we ever have a falling out... let's hope I never same them.