Okay, here’s my list:
1: Firearms
I don’t know if that fits into “setting” or not, but I loathe most “official” – and homebrew – rules for firearms. The reason? The designers try to do that “realistic”. Which means that: either it’s about early firearms, with slow-but-powerful balancing, or modern-and-overpowered firearms. When questioned, those people will simply say “firearms are overpowered in real life”.
Thing is, I Just Don’t Care. I don’t want “realistic” firearms. I want a Final Fantasy-esque world in which Lightning (with her gunblade), Cloud (with his Buster Sword), Vivi (with his magic) and Yuna (with her Tiny Bees pistols) can actually get along in a party and be reasonably balanced, and I’ll stab Physics as much as I need to get that. Just to a point, I’m a Physics student, and I don’t want to talk about my college while playing a RPG.
2: Fairies
Or better yet, the lack of it. D&D has traditionally avoided the term “fairy” like the plague, and use other names such as “Pixie, Sprite, Atomie, etc.” These names in themselves are fine, but I want properly named fairies. Right now I’ll play D&D, so I guess I’ll just say “All those races are part of the large Fairy species”. Oh, and their size. Right now those “little” fairies – along with races like the Brownies, Leprechauns, etc. – are too much big in my opinion. Really, 45 cm high Tinker Bell? No, that’s not right. Classic small fairies are the way to go. Okay, they might get stuck in a bag, but that’s part of the show.
Also, my gaming world isn’t exactly a fairy-tale and much innocence, but I like to have many different elements in my game, and a Disney Tinker Bell would be nice to have.
3: Class overlap
Comparing myself with other DMs of my area, I’m quite permissive – most of them are Core only in pretty much any game – and I actually like tons of classes. Problem is, with so many classes, some of them will have “conceptual overlap” problems – which are basically they both do the same thing on the world. When that happens, something must change or a class will be banned. The most frequent offenders are: Druid and Shaman. Basically, I don’t know how to have “priest of a nature god” and “priest of the nature spirits” without making one of them redundant or useless. It’s already hard to have Cleric and Shaman in the same game, but I can do it. Druid and Shaman, however, are not so lucky.
4: Alternative names for devils, demons, angels, etc.
Do you know terms like “Baatezu”, “Tanar’ri”, “Aasimon”, “Yugoloth”, and others? I hate them with the fire of a thousand suns. Basically, if you’ll have creatures with blatant references to popular real-world religions, name them right. The idea of renaming them just to appease the PC patrol is loathsome to me.
And by the way, I’m Catholic, and I like much more when the terrible and scary monster I defeated is called “demon” or something alike. I have no problem with that.