You can write a set of fantasy role-playing game (FRPG) rules without specifying a setting, but there’s a default setting assumed by virtually every FRPG. Moreover, some rules (e.g. the existence of plate armor, and large horses) imply things about technology and breeding in the setting.
Familiarity vs. strangeness is an important question for any worldbuilder to answer. What are gamers familiar with? That tends to be the default. J. R. R. Tolkien’s works (Lord of the Rings, Hobbit, etc.) are nearly a default setting for many, as in the dwarves and elves who are quite different from traditional stories of dwarves and elves. You could argue that the default setting is more Tolkien than it is medieval European, but he largely adopted Late Medieval European (1250-1500), so I prefer to refer to that.
The question is, do you want your ruleset, or your campaign setting, to follow the default? An early example of great deviation from the default was the wonderfully different world of Tekumel (Empire of the Petal Throne, and a few novels). A “different” FRPG might posit no monsters at all, perhaps not even elves and dwarves, just a lot of humans, yet never explicitly say so: if you leave out rules for monsters and humanoid races other than humans, you have a different-than-baseline setting, even if you didn't consciously make that decision. But be warned: too much unfamiliarity may make some players uncomfortable.
Are there baseline assumptions for science fiction? There seems to be so much variety, I wouldn’t try to pin it down.
Your Turn: Do you see the default setting as different that what I’ve summarized?
The Basics of FRPG
All FRPGs start with some assumptions built into the setting, some of them so innocuous that gamers might not even realize they're assumptions to begin with. For example the assumption that there are horses large enough to be ridden, even though for thousands of years of history, horses weren’t large enough for riding (the era of war chariots from about 1700-1000 BCE, and the era before that of infantry only).Familiarity vs. strangeness is an important question for any worldbuilder to answer. What are gamers familiar with? That tends to be the default. J. R. R. Tolkien’s works (Lord of the Rings, Hobbit, etc.) are nearly a default setting for many, as in the dwarves and elves who are quite different from traditional stories of dwarves and elves. You could argue that the default setting is more Tolkien than it is medieval European, but he largely adopted Late Medieval European (1250-1500), so I prefer to refer to that.
The question is, do you want your ruleset, or your campaign setting, to follow the default? An early example of great deviation from the default was the wonderfully different world of Tekumel (Empire of the Petal Throne, and a few novels). A “different” FRPG might posit no monsters at all, perhaps not even elves and dwarves, just a lot of humans, yet never explicitly say so: if you leave out rules for monsters and humanoid races other than humans, you have a different-than-baseline setting, even if you didn't consciously make that decision. But be warned: too much unfamiliarity may make some players uncomfortable.
Are there baseline assumptions for science fiction? There seems to be so much variety, I wouldn’t try to pin it down.
The Baseline
What ARE the baseline assumptions? In general, they are mostly late medieval (not “Dark Ages” (500-1000) or High Medieval (1000-1250), as FRPGs tend to be magic grafted to later medieval Europe. In no particular order here is a list of categories for baseline assumptions that I’ll discuss specifically:- Transportation
- Communication
- State of Political Entities
- Commonality of Magic
- Commonality of Adventurers
- Commonality of Monsters
- Length of History and Rate of Change
- Level of Technology
- Warfare and the Military
- Religion
- Demography
- Climate
Transportation
Wooden sailing vessels, late medieval style. In calm waters such as landlocked seas and lakes, galleys; in wild waters (such as oceans), small sailing vessels. River barges much preferable to poor roads and carts. And are there wonderful roads left by or maintained by an Empire (Rome)? See "Medieval Travel & Scale."Communication
Proceeds at the rate of travel, by horse or by ship. In other words, very slow by modern standards. Even as late as 1815, the Battle of New Orleans was fought after the War of 1812 had ended (in 1814), but before news of the treaty had reached Louisiana from Europe.State of Political Entities
Monarchies and lower level independent states (such as Duchies) ruled by “the man in charge” (very rarely, a woman). Nobles. States, not nations (the people rarely care which individual is actually in charge). Castles are so defensible that it’s fairly easy for subordinate nobles to defy their superiors. There are small cities (5-10,000 usually), not really large ones (over 100,000 people).Commonality of Magic
Magicians are usually rare, secretive folk. Few people ever see any manifestation of magic. In some cases the church or the government tries to suppress magic. See "The Four Stages of Magic."Commonality of Adventurers
Magicians, knights, powerful clerics, all are rare. 1 in 500 people? 1 in 10,000?Commonality of Monsters
Human-centric. Monsters are usually individuals rather than large groups. Intelligent monsters are rare. (Here Tolkien’s influence, the great orc/goblin hordes, often overrides European influence.) Undead may be common. Dragons are “legendary.”Length of History and Rate of Change
Slow pace of change of technology. Awareness of the greater days of a “universal empire” in the past (such as Rome), now gone. Technology changed much faster in late medieval times, than in Tolkien’s Middle-earth.Level of Technology
Late medieval, or possibly less. (Late medieval for the technology necessary to make full plate armor, if nothing else.) See "When Technology Changes the Game."Warfare and the Military
Wars rarely changed borders much (Late Medieval) - the great migrations have ended. Wars certainly aren’t national wars, the common people are spectators. See "The Fundamental Patterns of War."Religion
What we’re used to in later medieval times is a universal monotheistic church (Catholicism), though with foreign churches of different stripe (Orthodox Christian, Muslim, Hindu, Buddhist). But in games, more often the setting seems to derive from older, pantheon-based, religions.Demography
Density of population is low. Depends on whether the local area is frontier or settled. Cities are population sinks (high mortality rates). There may be stories of a Great Plague (later-1340s and onward in Europe).Climate
Temperate medieval European (more often, English (governed by the Gulf Stream)), with fairly cool summers so that full armor is not impossibly hot. (Imagine wearing full armor when the average summer high is 91 degrees F, as in northern Florida.) But winters are much less severe than in the northern USA. (Modern European climate is currently getting much warmer than in late medieval times.)Your Turn: Do you see the default setting as different that what I’ve summarized?
Or, more likely imho, it could be a variant of "one for the Church", with non heirs going into religion and magic for the families power. Inheritance would, largely, stay with the traditional route to power - the profession of arms. As for abuse of power, it happens. That's what adventurers are for 
