When you're facing a situation where there are catches to something, the deal breaker is that one catch that you cannot overlook and/or tolerate.
"Dealbreaker. She wanted to have sex in a Coffin"
(From the TV show castle)
"I loved Microsoft, but Windows Vista was the dealbreaker for me.
Jill: " It would have to be Arcade Fire."
John: "Wait...What? You don't like Arcade Fire? Thats a dealbreaker."
Billy: "Favorite Modern Warfare 2 class?"
Johny: "Well, I usually have the UMP .45 with a Silencer, Desert Eagle with a Tac knife, Marathon Pro, Lightweight Pro, and Commando Pro.
Billy: "Woah man. Way to be a noob. We aren't friends anymore, thats a dealbreaker."
Me: "Who's going?"
Vyatt: "Me, Barack, John, Hillary, Dennis..."
Me:"Hillay's a deal-breaker.."
Me: "You know, something came up at work..."