There's really not much at all that's a dealbreaker for me. I'd say general aggressiveness (especially towards humans) is something I couldn't deal with though. I could deal with dog aggressiveness (though definitely not something I WANT to deal with) but not human agressiveness. There are things I don't LOVE about certain breeds (shedding, drooling, barking, prey-drive) but I can deal with any of them. My ideal dog would be low-shedding, quiet, calm, good with other dogs, medium-large. (Thankfully I think Whippets fit that bill, and they are a breed I'm seriously considering). However, if I found a dog that shed a ton, barked a lot, and had high prey drive, but I just had a CONNECTION with the dog, I would deal. It's just the way I roll. For example, there are a lot of things about Huskies in general that I don't like (the shedding, the defiant nature, the JUMPING (I've never been able to train any of my huskies not to jump on guests), the fact that they can't be trusted off-leash, the fact that they have such high prey drive, etc. But I've absolutely adored each of the Huskies I've owned. There's just something special about them, and I'll always have a spot in my heart for Huskies. They may not be my 'ideal' breed, but I love them. I'm the kind of person that looks past 'imperfections' to see the perfections. Violet is perfect the way she is, even though she sheds everywhere, embarasses me when she jumps on company, tries her damndest to get out of the yard (or front door) to run the neighborhood, and won't let me have a cat. It doesn't matter. I don't LIKE those things, but I LOVE her. Chloe's pretty much my ideal dog, I only wish she was a bit bigger, but I don't even really mind her size. The descriptions I've heard of Whippets sound basically like a big Chloe, and I reeeaallly want to meet some.