|17th-century Dutch engraving of the Chinese imperial court. From Kircher's China Monumentis (1667)|
(Seriously, people - European armies started using cannons in the thirteenth century. They are at least as medieval as plate mail armour and highly developed feudal systems.)
I prefer the early modern period, partly because it's a period of history which I personally find fascinating, but also because - compared to the standard sub-medievalism of most fantasy settings - it is, paradoxically, both more familiar and more alien. More familiar, because it features more of the things that we have today: guns, printing presses, newspapers, and so on. But more alien, because those familiar things appear in a deeply unfamiliar context. We know where we stand with sub-medievalism: our characters thee and thou about the place, brandishing longbows and threatening to go off on crusades. The early modern period, by virtue of its greater unfamiliarity, retains a little more... edge.
So. What difference does it make if your game is set in an early modern setting, rather than a medieval one?
1: Everyone has guns.
I've already written about this at some length. It's worth mentioning that this does not mean that armour is obsolete: early modern metallurgy was good enough to produce metal armour that would stop a bullet. Armourers would prove this by shooting their breastplates with a gun, using the resulting dent as proof that a bullet wouldn't punch through it. This gave rise to the term 'bullet proof'. (You learn something new every day!)
2: Telescopes exist.
You've probably been assuming people have them in your medieval games already, haven't you? The telescope was an early modern innovation; not only did it permit huge advances in astronomy, it was also a boon to travellers and navigators. They were large and fragile, though, so you probably shouldn't let your PCs tie them to the barrels of their muskets in the hope of creating primitive sniper rifles.
3: Printing is commonplace.
Newspapers exist. Printed notices exist. Books still aren't cheap, but they're much cheaper than they used to be. Semi-literacy is increasingly the norm. Ordinary people who could never afford to commission a painting can decorate their homes with cheap woodcut prints, instead. Political radicals can circulate their ideas in pamphlet form. Your PCs can commission a printer to print descriptions of how great they are, complete with flattering engravings, and hand them out to potential employers. Printing is awesome. Embrace it.
4: People have access to stimulants as well as depressants
In most fantasy games, the only social drug characters usually have access to is alcohol. By the early modern period, however, they also have coffee, tea, tobacco, opium, and chocolate (which they used to make very dark, very bitter hot chocolate, prepared without milk or sugar, and used as a stimulant like coffee or tea). Your character smokes a pipe to relieve tension. Your character drinks black coffee to stay awake while she's on watch. When your character fancies a night out with her friends, she has the option of going to the cafe and getting all jittery and hyperactive instead of going to the tavern and getting sloshed, which enables very different forms of sociability. Your character craves really good coffee and really high-quality tobacco, and might do some very silly things in order to get her hands on them. Makes a difference, doesn't it?
5: People have access to painkillers
Well, one pain-killer, namely opium. (In ATWC, the serpent folk also have the thoroughly unhistorical ability to manufacture morphine, but their prices are very steep.) Opium use is completely routine. Forget what you think you know about opiates: your character swigs laudanum, or swallows opium, in exactly the same way that you take aspirin or paracetamol. Most people do this without becoming addicts, or suffering serious side-effects, although a character with the fatal combination of chronic pain and a low wisdom score may very well end up as an opium-eater. Given how frequently adventurers get hurt, opium is probably something they use a great deal, and it would never occur to them to think there was anything wrong with this. Sure, sometimes they get trippy dreams after a heavy dose, but what's the harm in that?
6: People might have access to phosphorus
This came in at the very end of the early modern period, although it's definitely around in ATWC. That means that you have access to a light source other than fire (although not a very good one), and maybe even primitive matches to replace your flint-and-steel. You can set fire to phosphorus and throw it on people, which will mess them up horribly - but burning oil is almost as good, and much, much cheaper.
7: Rich people have pocket-watches
These permit the exact measurement of time, and let you co-ordinate your actions with other people with much greater precision. (It's much better to sychronise your watches and agree to attack at the stroke of three than it is to have to rely on any of that 'wait ten thousand heartbeats' nonsense.) They also let you look like a really smooth bastard. I mean, a pocket watch. On a chain. Which your character can consult in between puffs on her tobacco pipe, while looking out for enemies through a spyglass and gulping down black coffee to help her stay awake, with three loaded pistols shoved into her belt and a musket lying at her feet. Tell me that's not cooler than yet another sub-medieval adventurer sitting around by a campfire with a sword across his knees, waiting to be attacked by 2d6 goblins. Of course it is. You know it. You know it in your heart.