One thing I will say - it's not about realism, as many claim. There's still magic, weird creatures, and other such things. Its a sense of versimilitude - that, while the normal rules don't apply, they make sense within the fiction of the game world.
As an example, much of the Forgotten Realms is saturated with magic, to the point where just about anyone can learn it, or even use a few spells - yet, few people have used it to advance past being shit-covered medieval peasants.
In the Black Company, wizards are feared as sorcerers and killers - because its the best way to survive being a wizard. Sure, you can become ridiculously powerful... But it generally involves doing terrible things, performing acts taboo across most nations, and then you catch the eye of other wizards, who are just as terrible as you are. Don't use magic - it only ends badly for everyone.
So, after my tangent, I want to make sure that my setting is accurate to the game mechanics I'm using, and vice-versa. So, there are many thing to consider when using E6 as a baseline...
- Magic is Limited - seems like a no-brainer, but with limited magic, comes much less in the way of inter-planar travel and knowledge of cosmology. To me, this means that myth and legend will play a larger part than in many settings, as there's next to no one who has seen the past, the outer spheres, or the basic planes. Even the classic "A Wizard Did It" things like magically-crafted dungeons, flying cities etc. can't be done (at least, easily). To me, this means that magic users will be rare, scholarly types, with Warlocks and other less-savoury types driven by the idea of bypassing magics usual limitations. It also means that I might want a Ritual Magic system, for the bigger, campaign changing spells.
- Magic Items Are Scarce - another thing to consider, most magic items cannot be crafted easily. A +3 sword is beyond the reach of mortal crafting under the usual system, and even a simple +1 sword requires knowledge of magic that most people simply don't have. To help remedy this somewhat, and to allow for some equipment advancement in lieu of usual advancement, I plan on porting over the Crafting system from the Black Company D20 RPG, wich allows regular craftsmanship to add minor bonuses to mundane items (instead of the usual Masterwork = +1). I'll post a little more about this later.
- The Demographics Change Drastically - in terms of NPCs, at least. One does not become a king by virtue of being the highest level dude around - most people stay as 1st-level NPC classes, with PC classes representing the few, more experienced men and women, rather than just bumping them up levels. With such a limited span of advancement, a 6th level character is the peak of mankind, and should be treated as such - not guarding the King's vaults. Once PCs reach 6th level, they're the top dogs - in terms of skill amongst men, at least...
- Monsters Are A Big Fucking Deal - imagine your standard setting. "Hey, random murderhobos, a gang of Ogres has taken up residence on the outskirts of the farmlands. Go rough them up a bit, eh?" A CR 3 creature is a pest, waiting to be exterminated. But, when everyone is a first-level warrior, it would take at least, what, ten men to one Ogre? A band of them might require a serious investment of military strength, and a horde of Orcs becomes a serious threat. Monsters become more than just part of the scenery - they become forces that shape and adapt civilisations. Don't expect to see too many humanoids up in Giant Country, and expect dragon attacks to take a serious toll.
- Combat is Deadly - linked to the above, somewhat, characters don't have a lot in the way of hit points, magical defenses, even healing. Each engagement should be planned in advance, tactically sound, and the very idea of a fair fight should send shiver a down a man's spine. I had considered adding the reduced Massive Damage Threshold rules from the Conan/Thieves World/Black Company RPGs, but really - shit's deadly enough as is.
- Resurrection Is Hard - barring powerful (and expensive) Ritual Magic, or perhaps divine intervention (yeah right), there's no way to bring the dead back. This makes assassination a useful political tool, rather than a nuisance (because what important person doesn't have a high-level cleric in their pocket?). For Clerics, there might be a chance to beseech your God for a miracle, but expect the cost to be rather... dramatic.
I hope to make a post about each of these over the next few days, focusing on each and discussing the implications and how I plan to implement each one. Stay tuned!