And in other caveats: I have very few absolute deal-breakers in terms of tropes. I know there are intelligent and sensible people--some of them right around here, in virtual terms--who will not read a book if it has vampires in it, or elves, or FTL travel, or something. That's their prerogative, but I'm having a hard time thinking of anything on which I'm like that. Also, I am clear that I am talking about things that bug me or throw me out of a story rather than grand universals of storytelling for all of humanity.
That said: I am a very, very tough sell on books where magic is a substitution or subtractive force rather than an additive one.
That is to say, if you try to tell me that science does not work in the fantasy world you have set up, or that a particular scientific force does not work, or that a particular technology does not work, you have just made it several million times harder for me to enjoy your fantasy novel.
"Science doesn't work" makes me think that the writer--or, if this is put in the mouth of a character, the character--has no idea what science is. Science: we make theories, we poke them, we adjust the theories. That is what science is. If you have a universe where this is not at all functional--where observing and theorizing and observing some more gets you nowhere at all--it is more or less the ultimate in horror stories for me. And it is in the direction of horror where I will put the book down and walk away quietly, and probably not pick up very many of your other works, either. But the thing is, you will almost certainly not be able to do this. Why do our houses and other forms of shelter stay up, and why do our clothes stay together? Science, people. Somebody said, "What if I prop this up this way, can I take shelter under it? No? How about this way? How about like this instead?" It's entirely possible that all such advances in technology--yes, knitting is too a technology--were divinely revealed in the universe you've set up. If you manage to do that consistently, I will be a) impressed at your extensive worldbuilding and b) totally uninterested in your characters, who huddle waiting for the next revelation or smiting. We don't generally score that a win.
"The electroweak force doesn't work"--okay, so actually nobody has ever phrased it this way, because the people who want electric fields not to function are not often the same people as the ones who care which fundamental forces have unifying theories and which are bizarre outliers. But there's a lot of "electricity doesn't work." Here's the thing: the electroweak force, charged particles and all? This is important stuff. This is really important stuff. Your nerves run on electrical impulses, but also, have you ever taken a chemistry course? Or a "physical science" course that talked about chemistry? Ever notice how those chemical reactions are talking about either ionic or covalent bonds? Do you know what those are? Those are the electroweak force in action--it is the same stuff that makes electronic devices possible. It is the very same stuff. What makes us able to digest things? What makes some materials ductile, some materials malleable, some materials hard and some soft? Electroweak force, my ducklings. You may not be able to trace "why can we make a sword out of this stuff?" to "how are the protons and the electrons in this substance interacting?", but other people can. And I am those other people.
So at that point you can jump in and say, "Well, but a magic force steps in and does those things in my imaginary universe instead." And I say, okay, really? It is a magic force that behaves exactly like the electroweak force on those things you-the-author don't really understand, but not those things you didn't really want? Your characters still eat and drink and breathe and have sex and whack things with swords someone has forged in a vague pseudo-medieval way, all in very recognizable ways, but motors and generators don't work? Isn't it a lot easier to just say, "In this world I've invented, no one has come up with motors or generators yet"? You run a lot less risk of me dubbing your magic The Force Of Authorial Convenience, I tell you what. Most people in the history of humanity--the overwhelming majority, really--have not independently formulated theories of electromagnetics, or even practical approximations of same, so "nobody came up with that" is a lot easier to defend than "the basic forces all happen to work in exactly the same way except when I didn't want them to."
"X technology just arbitrarily doesn't work" is even worse, because not only does it always come with a bunch of other things that work on the same principle, but it reminds me of ignorant people, many of them Young Earth Creationists, blithely stating, "I don't believe in carbon dating--I don't know, I just don't," as though radioactive decay is Tinkerbell and all the other fairies, and you can just stop clapping and it will drink poison for love of Peter Pan. As though carbon dating is completely separate from any other understanding we might have of the natural world. If you have gods running things, you can have them step in and zap whatever technology they don't like; that's fine as long as it has other consequences. If you have a world that is itself semi-conscious or in some other way directed, I can cope with that as well as long as it has other consequences. Having a god or some other principle who/that will squish your characters for figuring out penicillin really seems like it will have other consequences in the world. Having a character who notices that and tries to figure them out is a big step forwards for me.
This is beginning to look suspiciously like I am a sucker for following through consequences. See also: why I hate Veronica Mars Season 3.
When I had a brief beginning of this conversation (and it was my fault it didn't go further--sorry, guys, Minicon sort of intervened), my friends pointed out that if you are having a fantasy world at all, some things have to work differently than they do in our world. And that is absolutely true. But what I like best in fantasy novels is the feeling that there is something else, something more--sometimes something right up front and sometimes something just out of sight around the corner, but in either case more. Sometimes this is the sort of thing people mean when they talk about the numinous in fantasy, and sometimes it's a cool alternate-science. But when it starts being less than what we've got in this world, it appeals to me a great deal less, more than proportionately less--when it's this world minus, or this world with a cheap plastic substitution, I tend to end up disappointed and not much want to spend more time with that book or those books. Carl Sagan spoke derisively of people whose idea of religion was "the god of the gaps," trying to use the concept of the divine as some sort of spackle on inferior scientific theories. I'm afraid I feel that way about the magic of the gaps: it just doesn't shine for me if it feels like it's only there to patch the holes in half-assed worldbuilding.