@Xuc Xac, sktarq, et al.: People have said that they want us to drop the tangent, and I've received some PMs to that effect as well, so I'm not going to continue that line of discussion in this thread.
What I don't like is when players think it's most logical to ignore all of the magical stuff that's already there and try to duplicate real-life stuff like guns, nukes, and computers (or only use the magic to create the parts to make the modern stuff) while complaining about lack of progress. Why do the same old stuff when you can take advantage of things unique to a certain system? I can make an airplane or a tank in pretty much any system; you can't make immovable rod-based space elevators and prestidigitation-based Turing machines in every system.
To apologize for the earlier derail, some more of my setting turn-offs:
X) Mirroring good and evil as if they're just strict opposites and nothing more: as BoED and BoVD showed, saying that the opposite of something Good is automatically something Evil or vice versa, or saying that something is Good just because it's only applied to/used against Evil things, just doesn't hold up. Sure, in D&D alignment is fairly black-and-white, but you don't need a paladin of slaughter for every paladin of honor, or a talisman of good for every talisman of evil, or the like.
I'm not talking just about opposed magic or themes or whatever, since those are more nuanced; blackguards are different enough from paladins to have a place, and the Light Side and Dark Side in Star Wars aren't just mirror images. It's only when things get to the level of BoED's "Poisons and diseases are evil, but ravages are A-okay because they're poisons and diseases for evil people!" or Star Wars's "Force Lightning is totally evil, but Electric Judgment is totally good, because Force Lightning is blue but Electric Judgment is green!"
X+1) Simplistic alignment associations for gods. Gods of death are usually evil, gods of nature are usually neutral, gods of light are usually good, and so on, and all of the gods have fairly tight themes (gods of death tend to also be gods of icky/spooky things too, and gods of knowledge tend to be neutral and uncaring). There are some exceptions in published D&D settings, due to sheer numbers of published gods if nothing else, but not enough homebrewed settings have gods like, say, Pluto/Hades, who was good of "death" (the underworld) and also wealth and trade, due to associations of wealth and death with being underground...or gods of both death and nature who are Lawful Good and all about working with the pack/flock/clan, dying for things greater than yourselves, death being a natural part of the cycle to be cherished instead of mourned, and so on...or a pair of sibling gods, one of civilization and cities, one of trade and travel, who are both evil and have domain over smog, shipwrecks, and other bad aspects of their portfolios.
X+2) Monotheistic churches in polytheistic worlds. Too many religions in polytheistic 'brews have churches with Pope-esque leaders, a regimented hierarchy, established places of worship for laypeople, and so on. Sure, D&D religions started off as Catholic Church lookalikes with their paladins and clerics and restrictive domains/spheres, but it's not that hard to have religions that have decentralized authority and/or lack temples and shrines and/or worship some number of gods between "one or two of them" and "all of them" and so forth.