To me, deal-breakers are related to what people expect of the game. I'll use myself as an example: I see lots of role-playing systems as being able to deliver a good experience in terms of fantasy gaming. I once played in a fantasy campaign that used Vampire: The Dark Ages for the basic rules. Personally, I believe Luke Crane's Burning Wheel is able to deliver an experience in fantasy gaming that D&D is not able to replicate. So, why do I play D&D?
- I play D&D to kill beholders who can petrify/disintegrate with an eye ray.
- I play D&D to have some characters that are lawful, some that are good, and some that are
necessarily lawful good (yes, I'm talking about paladins here).
- I play D&D to get high elves that are different from gray elves, demons that are different from devils and blue dragons that are different from silver dragons.
- I play D&D for the great wheel cosmology, and I couldn't care less for the fact that the elemental plane of fire is not a good setting for my adventures.
So, all of those things are deal-breakers? Yes, they are, because If I'm going to play a game where silver dragons are evil, blue dragons are good, elemental planes don't exist and gray elves are called eladrin, I may as well simply play a game where becoming more powerful doesn't mean becoming better at melee combat, and being wounded in combat actually means something before you go down below 0 hp.
To some people, D&D is generic fantasy gaming: build the game you want, as long as it has dungeons and dragons in the mix. I'm not one of those, and that's why I have my deal-breakers. I don't want to push WotC in the direction I'd prefer, but I see lots of games with better building blocks for a "build the game you want" model. I like D&D because a specific kind of flavor is supported, and departure from that flavor was one of the main reason I kept away from 4E, and will do the same for 5E if any of my deal-breakers indeed appear broken.
Cheers,