For me, the deal breakers came up as we got to know eachother on early dates as the conversation flowed. I often knew within 3-4 dates if we were at the "dealbreaker" conversation stage or not.
What was funny though is that I discovered as I got older that things that I once thought were deal breakers aren't anymore. What was a deal breaker for me 10 years ago, isn't necassarily a deal breaker now. I've changed and what I want in life has changed so naturally, my deal breakers have changed too.
For instance, in my early 20's, smoking was a deal breaker. I wouldn't even go on a date with a guy if I knew he smoked. Well... My Hubby is a smoker. I knew he was a smoker before we even started dating because we were friends first but somehow, as time passed, and as my feelings for him grew, that didn't seem so important anymore. He had so many other good qualities that I was willing to get past the smoking. He has, since getting married to me, quit smoking though because he knows I hate it. He still slips up occasionally but his consumption has gone waaaaay down since we started talking about having kids.
Though, I am also willing to compromise on having kids too now to be honest whereas previously, a man not wanting them would have been a deal breaker. Just in the last few years, I've realized that while I'd like to have them, I'd also not be hearbroken about it if it didn't happen. I just don't feel the same way about it as I did before as my life is really already very full. DH is of the same mindset. We want them and we think kids would enhance our life, but we don't think our lives will less full without them.
So, basically, as my life has changed, so have my deal breakers. Though, some things I will never change my mind on. If I were still single... My deal breakers would be: cruelty to animals and waitstaff, road rage, being a slob, and if the person I was seeing wanted me to give up theatre... those are some issues that I'll never compromise on. And the first two I would probably get a sense of on the first date without having to even talk about it.