Personally I wouldn't mind having access to firearms up to and including cap and ball revolvers and rifles (i.e., roughly up to the mid 19th century). I would tend to be disinterested in metallic cartridge weapons, but I don't think D&D is that far removed from the Old West (American West, that is) in a lot of ways. The Good, The Bad, & The Ugly could easily be a D&D adventure.
As far as cannon, mortars, and other large firearms, I'm less interested in that outside of naval combat or siege warfare. However, I think cannon might be important to naval warfare. I think most D&D players think of 18th century naval combat when they think of D&D naval combat. Very few people think about grapple ships and marines.
I don't mind weird science or steampunk trappings (note: I'm not interested in discussing the de jure meaning of those terms, TYVM). However, I prefer that most of the trappings stay as terms like "arcana," "artifice," "alchemy," or "astronomy" (maybe even "astrology"). I don't think D&D worlds should have proper science in them even if they do have things that we would call the products of technology or engineering or science. Simply put, I want the natural world to be mostly inexplicable and mostly the realm of unknowable knowledge. The game is still a fantasy game, and it should not attempt to explain how things work. Like other fantasy stories like LotR or Conan or Star Wars, things work and that's all there is to it. If you're questioning it, you're doing it wrong because that's not what the story or world is about.
In most D&D worlds I've run, I create an equivalent of Martianus Capella's De nuptis. A basic education means schooling in the seven liberal arts: the Trivium (Grammar, Rhetoric and Logic), and then the Quadrivium (Arithmetic, Geometry, Astronomy, and Music). Music seems out of place, but they used it to study harmonics of vibrating strings, and so it was one of the very few applications of the sparse academic knowledge they had. In European history, this was the basic education at a medieval university. The Trivium taught you how to think and formulate your thoughts and present them to others. The Quadrivium taught you knowledge about the world, how to measure and explore the world, and the way things worked (or, rather, the ways very learned men had guessed things worked). Completing the coursework of these seven liberal arts made you a Bachelor of Arts. After then continuing to a Master of Arts, you were then qualified to study for bachelor's degrees in one of the three "higher faculties": Theology, Medicine, or Law. In most D&D campaigns I run, I typically change that to four higher faculties: Religion, Arcana, Medicine, and Law.
That doesn't mean that these people weren't experimenting, of course. It's this kind of education that allowed Theodoric of Freiberg to correctly explain how the rainbow works in about 1300, and he did it by experimenting with glass spheres and water filled flasks:
(Relevant bit starts at about 2:20.)
So, I tend to favor D&D settings with 18th century weapons, 17th century naval combat, and no more than about 14th century knowledge. It should be explicitly proto-scientific.
