I was looking through GURPS' Dungeon Fantasy (which is basically 'play D&D with GURPS rules'), and this bit struck me:
"Consequently, it has some unusual conventions, including a quasi-medieval setting full of modern social developments such as sexual equality, sprawling metropolises, and a cash economy; a technological mix whose only rules seem to be “preindustrial” and “no gunpowder”; casual acceptance of magic and holy miracles; trade guilds for thieves, wizards, and even assassins; and a culture that recognizes “adventurer” as a career choice.";
a) Social conventions vary widely across the campaign world. The greater prevalence of sexual equality in a modern sense compared to the historical real world is easily explained in setting by the fact that women are just as capable (and possibly even slightly more capable) spellcasters as men, and as such have obvious value outside of being mothers that they didn't have in a world where making war highly depended on upper body strength alone.
b) Sprawling metropolises existed outside of Northern Europe even in antiquity and are as such not ahistorical. Generally higher quality of life compared to the real world can be explained by magic and related higher levels of technology and information sharing compared to the real world (in a "magic is technology" paradigm).
c) The chemistry of the setting is vastly different than real world chemistry is it assumes "earth", "fire", "air" and "water" are actually elements and so something like "sodium" or "oxygen" are actually molecules made out of those elements. As such, chemical compounds can be expected to be wildly different from the real world. In the setting, there are no known explosives more powerful than gunpowder and more stable than warm nitroglycerin. Attempts to manufacture bombs and even guns have occurred historically, but have always resulted in the manufacturer blowing themselves up in the medium term as bombs rarely last more than 24 hours before self-detonating, and attempts to use guns resulted in chain reactions of explosives when soldier attempting to reload was subject to a spark that detonated his own stored powder, which in turned detonated the stored powder of those around him, devastating the entire army in a one sided defeat.
d) Casual acceptance of magic and holy miracles is not ahistorical. What might be different is how subtle and reliable these daily miracles are generally perceived to be. See "magic is technology".
e) Trade guilds for wizards are entirely reasonable if wizardry is perceived to be a trade.
f) Trade guilds for criminals are really just criminal organizations organized by the criminal according to prevailing cultural paradigms. They are fundamentally no different than criminal gangs, families, cartels or corporations - it's just the people in the culture are predisposed to refer to their organizations as a "Guild". Thus a "thieves guild" is really no different than the Colosimo mafia family, or the Sinaloa cartel, or Yamaguchi-gumi yakuza family, it's just guilds are the dominate cultural paradigm for corporate activities. Note that in the setting many of the guilds would also be properly described as cults, revolving around the veneration of one or more deities that patronize criminal activities for whatever reason. It's not unknown for the thieves' guild to achieve some sort of official status within the setting, that allows it to operate with some official protection, but it is rare. Usually when it happens it happens because the State recognizes the underlying cult and wants peaceful relations with the deity that it represents and may continue if it feels that the deity provides some significant aid. For example, if the current regime in the nation was established through civil war, and the cult aided the rebels, then the guild may enjoy some level of official or unofficial recognition. The resulting extortion racket can be rationalized by even those that resent being shaken down as a tax or tithe little different than their other religious duties. And so forth. Much more often though, organized crime is tolerated in an unofficial capacity because it has penetrated the upper ranks of the aristocracy and is bribing, blackmailing, or actually ruling the society in some fashion.