Pretty much nothing would, for me.
Short of, y'know, high-tech stuff when there's no precedent for it in the setting, unless the group has come to an understanding beforehand (like when playing in a homebrew or other setting they're not as familiar with).
But I'd have no problem with that kinda stuff if playing in a standard setting, where I could expect such things to have potentially come to the plane via spelljammers, time-traveling illithids or aboleths or whatnot.
If I'm playing in Greyhawk or Forgotten Realms or Eberron, I'm not going to care if firearms show up. Uzis and RPGs and tanks might rattle me when playing in such a setting, but American Civil War era style firearms and cannons won't. And I'm not likely to quit over it (unless the DM is being a total arse, expecting low-level PCs to survive against main battle tanks and machine gunners and such).
Dinosaurs, space aliens, robot ninjas, any of that stuff, bring it on.