I feel that one key area where the game is very US-centric is in the issues of social equality and social mobility. Sure, D&D has kings and lords and nobles of all stripes, but this is really a very thin veneer over a fairly classless society, especially where PCs are concerned.
I agree, but I don't so much find this an American bias as part of the general 'assumption of convenient anachronisms' which has crept into modern gaming on the simple basis that it is convenient and familiar. Even if the DM imagines a non-familiar world, there is still the problem of getting the players to understand the world in which their characters live and act accordingly.
Typical gamisms:
The PC's stay in inns that are very much like hotels, rather than much more like staying in people's homes.*
The PC's order food off extensive printed menus, rather than being served what the host has or simply asking for particular items to be served.
Coin is common and plentiful.
Everyone speaks the same language.
Weapons are freely available and may be carried everywhere. Full plate may be worn into a free town, and no one blinks an eye.
Persons (especially PCs) have freedom of the roads, even when armed.
PCs never pay taxes.
Finance and the instruments of finance are known.
Destitution and poverty are rare, except for when they show up like 'bad weather' to signify the presence of the BBEG.
Gender equality is assumed.
Racial equality is assumed.
Xenophobia is rare.
Corruption is rare.
'Good' aligned societies have modern sensibilities about cruelty in punishment, slavery, libertarianism, and generally resemble modern liberal western democracies in every feature but name.*
Well defined nation states.
Almost everyone is literate.
Professional national standing armies.
PCs have freedom of speach.
Goods can be purchased on demand off the shelf from a large existing stock of such items.
Rank doth not have its priviledges. There is a presumed meritocracy and freedom to change social classes.
*Those might be American biases, or at least Western European biases.
Violate just about any of those gamisms in your world, and not only does it become more complex, but the unspoken assumptions of your players will have to be continually corrected during the game and especially in the early sessions when you'd really rather get the game moving.
As for 'Legality of Weapons' issues, they are typically missing from all fantasy campaigns, but show up in the rules of all or most modern campaigns regardless of whether the game is American or not.